I just bought the game. I played BG1-2+IWD1-2 several times (haven't touched PoE, DOS or DA). I was thinking about playing the game for the first time on the easiest difficulty, just to experience the story. My intended party is a group of goody two-shoes:

Amiri (Minsc)
Octavia (Imoen)
Valerie (Tank)
Linzi (Bard)
Tristian (Healer)

What main character class would you recommend? I was thinking about Paladin (Champion) or Fighter (also tank).

Could you recommend me some builds? Some easy to use (auto-attack) builds would be nice.
How easy is it to get these characters into the group?
Should I know something in advance? Should I avoid something?

I am not familiar with the game, I just googled the companions and picked the ones who are good aligned (+Amiri).

You can have anybody in your party regardless of alignment

The pathfinder system is most like that of IWD2 (both are D20 systems)

MC as second tank would be a good choice. You can freely multiclass without fear of XP-penalties like in IWD2.

It probably won't be as important on easiest difficulty but buffing can make a huge difference

Also important: AC is evrything or nothing. Either you get up close and personal and have as high AC as possible or you keep away from danger and mostly ignore AC. Having some AC and getting into melee is just gambling with the RNG.

Amiri: She's always in your group when you leave for the stolen lands
Valerie: She sides with you if you pick the 'lawful' option. (otherwise you get Harrim)
Linzi: She sides with you if you pick the 'good' option. (otherwise you get Jaethal)
(you can still get Valerie and Linzi later)

You get Tristian after completing the temple of the Elk. But if you went after your rival first, he'll be recuperating (and unavailable) until the end of the chapter instead)

You free Octavia from the Technic League camp. You'd probably be lvl3 or so by the time you reach her.

I have seen a video that suggests druid, monk or paladin as the characters class, the logic used was none of the NPCs you meet are initially those classes. Since you can easily multiclass an NPC I am not sure how solid that thining is. A custom character build specifically for those classes would probably have a better optimized stat stpread, where as NPCs get what they get for stats so some multiclasses wouldn't be optimal on them.

I am not far in so I can't say if the you don't meet any NPC that have the mentioned base class is accurate.

On the easiest difficulty you can play whatever you want. Don't worry about it; play as you like, may be fail a couple of times, but go through the game. And then you will either figure out some builds for yourself or move on.

My party is the same as yours except that I replaced Amiri with Ekundayo (who is also good-aligned) when he became available (he's as good or better than her at dealing damage, especially if you equip him with the "Devourer of Metal" longbow, and his wolf is effectively a whole extra party member). My PC is a magus.

Personally I recommend Sorcerer. However, on easy mode, you can play pretty much anything. Well probably.

There are a few parts where you want high Diplomacy. Also being able to turn yourself invisible might actually get you out of some fights. Get a Quicken spell rod (there is at least one) and use Vanish or Invisibility. If you combine this with some Stealth skill, you can possibly escape harm or do other cheesey things (there's an amulet you can get in Varnhold which is excellent for this purpose). Note that I'm talking about parts of the game where you may be forced to fight solo or with a limited party. There are a few.

Here's how I started:

Level 1: Amiri, Linzi, Valerie
Level 2: Amiri, Linzi, Valerie, Octavia, Harrim
Level 3: Amiri, Linzi, Valerie, Octavia, Tristian
Level 4: Jaethal, Linzi, Valerie, Octavia, Tristian
Level 5: Jaethal, Jubilost, Valerie, Octavia, Tristian
Level 6: Jaethal, Jubilost, Valerie, Ekundayo, Tristian
Level 7: Jaethal, Jubilost, Valerie, Kanerah, Tristian
Level 8: Jaethal, Valerie, Nok-Nok, Jubilost, Tristian

That's as far as I've got. My character is a Magus (basically ineffect's Sword Saint build). I did a partial play-through earlier with a Ranger. With that character I found Ekundayo to be a bit of overkill.

Of course, you can really do whatever works for you. If you're going to customize your companion builds (which I did) you may want different synergies. At one point I found I had a bunch of companions, none of whom had access (yet) to certain spells I felt I needed. Web, for example.