More difficult to play than with a standard party but should be doable if it isn't your first game.

The monk/druid doesn't have a lot of class chemistry; I suggest ditching one or the other class and focus that character on just one. Monk and Druid both do better when they aren't sharing levels with another class. You already have a paladin and a barbarian, which both serve a similar role to the monk, so a druid might be slightly preferable.

There are several characters with level adjustments. Consider whether you'll be using the benefits of the race enough to justify not having extra class levels.

Other than that, you have a front liner, a divine spell caster, an arcane caster, and a rogue.

I think the idea was just to get one level of monk for the AC bonus from wisdom and take druid levels the rest of the way. Personally, a pure druid stays at the back and doesn't need the AC bonus from the monk level.

Well - yes. Also, I found you need monk as a class requirements in some quests.

Some quests? Nope, just one and that quest must be the 8 chambers of tediousness which your half-orc barb/fighter will be able to handle, no monk needed. In this game, monks only start to shine right at the end of the game, so I wouldn't bother unless you're playing HOF mode. However, its your game so, your choice ;)

Not having a monk won't lock you out of anything so feel free to skip it if the 8 chambers of crappiness is your only reason for taking one ;)

Edit: Will you be using your druid on the front lines? If so, there is a case to be made for having a 1 monk / X druid character

Well, I'm not against using druid in front, but he/she is my backup fighter and caster - like Jaheira is in BG. I will definitey skip monk.

Ah. That makes sense. I was reading it as half monk and half druid levels.

Good idea dropping the monk level. In IWD2, it would be a good idea to see what spells you are planning to use as the effectiveness of some spells are tied to the number of levels you have in that class. This is why it isn't really a good idea to mix other classes with casters unless you know what you want from that character. If you are playing IWD2 normally with no mods or level squatting, Human characters will have 16 levels to play with, less for the more powerful races depending on the level adjustment

For druids, if you are planning to use evocation spells, it would be best to keep him/her as a pure druid (though one monk level won't hurt much) to get the most power from those spells. However, a lot of druid transmutation spells don't need a high number of druid levels to be effective. It all depends what you want from characters. Here's my recommendation for druids.

If you want a backup fighter and a good caster, use a pure druid. Boost WIS on level ups
If you want a good fighter and a back up caster, use a 4 fighter / X druid. Only use transmutation and healing spells. Use a two handed weapon. Boost STR on level ups.

All these level adjusted races are going to be nasty, the Paladin especially. I ran a Paladin/Sorcerer in an IWD2 playthrough and it always felt like it was trailing behind the rest of the party, and that was with a human. Based on my experience, the Rogue level is unnecessary. Just stick with the Transmuter. Wizards will have a massive intelligence bonus, which applies to rogue skills and largely eliminates the need to have any levels in the rogue class at all. Given how weak Rogues are in IWD2 (for whatever reason, the class was nerfed from the D&D rules and sneak attack is practically useless) I'd actually say the class should be ignored entirely. Pretty standard combo, and for very good reason. It's one of the most powerful race/class combos you can get in IWD2. This guy will be doing the heavy lifting for the party. The others have already covered this well, but I'll reiterate that Monk is a very finicky class. Its fighting style is incredibly low-damage, making it dependent on having a very high strength score to be functional.

There is a case to be made for taking a rogue level for a thieving skill boost and bow proficiency but I think it would be better to make the caster an elf instead for using bows. Elven enchanters with skill points in speech skills are a personal favorite. But yeah, I know the OP wants a transmuter for the drider area.

You could also just spend a feat to get proficiency if you really want it, and that's much less expensive that investing a whole class level. Elf is definitely the best approach if you want a wizard who wields a longbow; those free weapon proficiencies are really convenient at low levels.

Yeah, totally agree with you, wizards, especially enchanters who can ignore evocation and elemental damage feats, get enough feats to be able to become proficient in bows anyways. It's just for a wizard, I would rather place early game feats into the spell schools then one into rapid shot as early as possible. I don't actually like the idea of 1 rogue / x wizard builds myself but some people do.