I have thought a bit about the things here.
I know the DND2 and DND3 rules relatively good but there are many games and sometimes its hard
to remember which rules are different in wich game, even when they use the same system.

IWD1:
unlike BG and NWN you don´t have a specialist wizard that lose only diviation.
That means I will take a general wizard. (I think IWD2 is the same)

With an exp limit of 2.7mio dual classing makes some sense.

So this is my new party suggestion:

-fighter3 -> mage (A mage that can use a bow OR a shield, bows are the best ranged weapons because of their high fire rate)
-Bard (now he knows more than one song, that makes him even better)
-fighter/thief (you are right. a pure thief will have more skill points than he ever needs.)
-ranger/cleric (maybe not as overpowered as in bg2 (dual wielding chrom fayr and fail of ages self buffed, but should be good enough)
-fighter 7 or even 9 ->druid (druids have a bizarre exp curve, very fast in the beginning and very slow in the middle. That makes them better for dual than for multi class, I guess)
-paladin (tank)

question 1: in DND2 a thief can only backstab with a weapon that is allowed to thiefs, but in DnD3 you can make sneak attacs with any weapon, right?

In that case the fighter/thief will get a bow, a long sword for backstabbing and a great sword for fighting. As multi class you can have only two profiency points per weapon.

question 2: The concept of DC (difficulty class) does NOT exist in DND2, right?

If a spell has an effect is only dependant on the targets saving throw and NOT of the casters stats or level.

IWD2:
Question 3: IWD2 has NOT the feat "able learner" (learn cross skills for 1 point) but if you have a class skill x and you gain a level in another class you can raise x to character lvl + 3, right?

Question 4: What is the maximum amount of classes you can have?
Since there are no prestige classes more than 3 makes not much sense I guess.

Question 5: When you have levels as monk or paladin and you take another class that is NOT the class of your order, you can never take paladin/monk levels again but you will keep all abilities you have, right?

new party suggestion:
-rouge 1, rest wizard (same as before)
-paladin 1, rest scorcerer (sorry, I like pallies too much)
-rouge/fighter (ranger feats do NOT qualify for improved dual wielding )
-Bard/barbarien (str based fighter + buffing)
-fighter/paladin (tank +dd)
-fighter/cleric of tempus (does one lvl of fighter make such a big difference? usually people take 4 for weapon specialisation, as you have written this one should be more fighter than caster)

Notes: - I like paladins even though they have the highes stat requirements
-I like clerics more than druids maybe because the thought of becoming an animal is somewhat scary for me.
-Usually I am more focused on combat than magic. Having 2 primary arcane casters in my group is a lot for me.
-If it goes up to lvl30 one lvl of something else should be ok for a caster.
-I am not sure if I should use ELC classes on my first time in this game, but aasimar and tiefling stats are just great. Drow are really cool but I think being always two levels behind is too much.

Correct. In the original release of IWD, I believe there was a school that had only Divination as the opposing school, but this was changed in a patch (and the GOG version is patched to the latest).

Just make sure the Fighter that you're switching to Druid has a Neutral alignment as well as at least 15 WIS and 17 CHA. Otherwise he won't qualify to dual class. If you find you want him to level in Druid fast and regain his FTR abilities, you can camp out on the level of Dragon's Eye where cold wights spawn and farm them for quick XP. (They'll interrupt your rest almost every time you attempt it). This should be pretty close to the area where you'll get enough XP to dual class anyway.

As for bows, I prefer slings for any class that is using a shield as well. It's just a pain to keep switching out the shield and weapon and replacing it with the bow (there's no hot key to do that, so you have to keep accessing the inventory). If the character also has high STR, then you'll also get the extra STR modifier to the damage dealt.

Yes. As long as you're flanking you get sneak attacks in IWD2. The damage is calculated differently as well (it's not multiplied like in IWD 1, rather you get extra damage dice).

I believe this is correct, if memory serves. If kmonster is around, he can confirm this.

Correct, although if the skill is not a skill for that particular class (i.e. if you're leveling as a Fighter and have previous Wizard levels, raising a skill like Spellcraft), then the skill will still cost you two skill points to raise it one rank.

I'm not actually sure if there's a limit on this as I've never tested it out. But yes, more than three would do far more harm than good, I would think. Yes. If you stray from the path of that particular class, you're barred from leveling in it again. But you do keep the special abilities of the class.

Re the PAL/SOR: I don't see the point of the Paladin level with this character, as the bonus to saves is of limited value to the caster, since he won't be the focus (usually) of spell attacks. And you'll be delaying your spell progression even further (and Sorcerers are already behind Wizards in gaining new spell levels). But it's certainly not going to cripple the character either.

Re the Bard/Barbarian: again, I'm not sure I see the value here. You're delaying your Bard's spell progression by taking levels in another class (and they're also slow to gain new spells), as well as delaying the Bard Song progression. I'd go with pure Bard here. You should have good tanking value with the Pally/FTR, FTR/CLC, and even the Rogue/FTR. The Bard can cast some buffs, toss a couple offensive spells, plink away with a ranged weapon, and sing (do keep in mind that if the Bard does anything other than move once the Song is activated that it will turn the Song off. The Song effects linger for two rounds after it switches off though).

Re the FTR/CLC: you can certainly go with 4 FTR levels for Specialization if you want. This will actually net you another extra feat from FTR bonus feats, which is never a bad thing. However, I just choose the single level so I can get my spell progression moving a little quicker to get access to buffs sooner (and make them last longer).

The Druid shapeshifting can be pretty handy sometimes, but every time I've used a Druid in IWD2, I've pretty much always forgotten to use it. (LOL) I prefer Clerics as well for this game.

2 casters is excellent for a party of six, especially if you're focused on melee combat. You get a good array of very useful spells and buffs, but you don't get overwhelmed with constantly having to tend to the spell casting. The Bard's spells are a great bonus as well (I like to give him Dispel Magic, Invisibility, Haste, and Grease as a few standard issue spells).

You won't reach level 30 by the end of the game. You'll hit level 20 (19 with -1 ECL characters). The idea is to export your characters and start again in Heart of Fury mode so you can get to max level. I've actually never played HoF mode myself. Doesn't really appeal to me, even though I like a challenge.

I love both the Aasimar and the Tiefling, especially for the roles of Sorcerer and Rogue/Wizard. But if you're still planning on adding a level of Paladin to your Sorcerer, I wouldn't go with Aasimar. Your progression is delayed even further if you do that.

Anyway, that's my 2 cents. Hope it helps.

I would just suggest that you are waiting too long to dual-class the druid. You will be without the healing power early (though you do still have your ranger/cleric), and it will take a while for the druid to catch up when you make the dual change. I would do all my dual-class changes from fighter after 3rd.

It is not like BG2 where you can start with a brand-new 7th level party and have some easy fights early (along with cheap xp from scribing spells and successful thieving) to zip through those transition levels. You will have to suffer quite a bit of time in the transition in Icewind Dale if you put it off too long.

confirmed. There's no limit besides the maximum number of classes which is available for your alignment (9 for alignments allowed for druid, 8 for the others). But it's not worth it. Actually if you play cheating the average character level of your party will be just below level 17 at the end of IWD2, add 10 levels for HoF mode. ECL will barely change anything since the dynamic combat XP calculation will even out most of it.

One advice for IWD2: If you want to do physical damage you need maxed natural strength, tickling your opponents with many, many HP and maybe even damage reduction to death is ineffective, slow and boring.

For comparison:
You reach fighter level 3 early in chapter 1 and if you dual to druid/mage it will be complete late in chapter 1.
You reach fighter level 7 late in chapter 2 and if you dual to druid/mage it will be complete early in chapter 3.

The game has 6 chapters and 2 expansions.

One more question:
How big is the need for a tank char in both games?
I mean somebody in heavy armor, shield and other ac boosting things.

In NWN2 and BG1+2 it looks like the enemies attack the char they see first
or that is closest to them.
A tank was very useful when fighting an armee of goblins but i can remember in BG2 the fight with the red dragon.
He hit my warrior with a attack throw of 2, even though the warrior had the best equipment you can have at that point.
After the dragon pushed my chars away he went to the one closest to him. My party was standing in a circle around him so his breath did not hit everyone.

The tank char would be the paladin (fighter/paladin in IWD2) which makes some sense from a role playing point of viev too.

Since IWD1-2 are both heavy on melee combat, I'd say there is a great need for a tank. (or even two) Of course if you're familiar with the game you can experiment with more "exotic" party combinations, but for a first playthrough at least one tank is advisable to avoid frustration.

As for enemy AI, it is the same as in most IE games, so monsters usually attack whoever they see first and keep pursuing that character unless another one is persistently hitting them.

I find that, in IWD 2, the AI will sometimes be smart enough to target your mages. But overall, yeah, it's pretty much a 'chase the first thing I see' type of thing.

I haven't played through IDW2 yet.

In IDW 1, the difference between my tank (with the best AC) and my next best character in terms of AC was a big, big difference, and very noticeable in terms of survivability. If I were to play through again, I would still pick one character to be the tank and stack that character with the best AC possible.

There are a lot of combat situations where you can use your tank to draw all the hostility.

There are also a lot of combat situations where your party gets flanks from multiple directions, and situations where it does seem that the monsters completely ignore melee characters and go for spellcasters, and situations where monsters will switch off from characters with good AC and go looking for more hittable targets. Those are the situations where I would look at spells / potions / abilities to help the party through the combat.

You don't want to rely on spells and abilities for every combat, because finding a good place to rest is sometimes problematic. So I think it's good to have a tank to grind through as many combat situations as possible, and save spells and abilities for other situations.

Well, in NWN2 you can get very high Armor classes with the right class combinations.
A monk, duellist, swashbuckler, rogue would add his dex, wis, int and a robe to ac and
could attac good too.

IWD2 does not have prestige classes so you have to relay on dex and armor.

And in DnD2 (IWD1) all you can do is creating a a fighter, ranger, paladin with dex 18 and hope to find
good armor and shields.

There are other things boosting your ac but usually they don´t stack.

The only other tank I can think of is a fighter/mage buffing himself before putting on his plate mail and shield.
Not sure how long spells like mage armor and protection from whatever last in DnD2.

If this is good he can replace either the bard or the ranger/cleric and if he replaces the ranger/cleric, the fighter->druid would become a fighter->cleric instead.

Another thought about IWD2:
My tiefling fighter/rogue might be better if making sneak attacks with a bow.
When he stands in the back and shoots monsters that attac somebody else (the tank would be best) with rapid fire, he adds the damage of one arrow per sneak attack dice and has little need to move during combat.