Sikret, the maker of this mod, would strongly recommend a "Vagrant" for your protagonist. There are several Vagrant specific quests and items, so if you're looking to go the ranger route, Vagrant will add quite a few nice perks.
In terms of overall party balance, you need to have a lot of melee power. Don't even bother rolling up an Archer, or similar class that focuses on missile attacks. Not when you're playing Improved Anvil, at least.
You probably want to include at least 1 single class fighter (I usually change Minsc to a berserker) to ensure you've got someone with the best armor, shield, and grandmastery in their chosen weapon. This person becomes your meat shield who forms the backbone of your party.
I recommend at least one other really strong melee member. You might consider Valygar for this role. It might seem like a lot of rangers, but rangers are rather godly in IA, so just roll with it
. Valygar gets 2 *really great* item upgrades, and has access to other ranger specific perks in the game.
You should probably shoot for 2 strong healers, and clerics are preferable to druids largely due to their greater restoration spell. A multiclass ranger/cleric isn't a bad choice for one of those slots, and someone like Anomen makes a great primary healer (and a really good fourth tank).
You also should shoot for at least 2 characters capable of casting breach. You will be casting this spell A LOT, and one caster of the spell just isn't enough in my opinion. Nalia is great in IA due to her improved ring option, and her thieving skills are redistributed in IA to make her better at finding traps annd opening locks. You'll want to have someone with at least rudimentary theiving skills.
I hope that helps, it's a lot to fit in. Just to give you an idea, here's what my current party looks like:
Vagrant (Protagonist)
Berserker (Misnc)
Ranger/Cleric (Jaheira)
Stalker (Valygar)
Cleric/Mage (Aerie)
Swashbuckler10/Mage (Nalia)
It's got very strong melee. 1 good lead mage, with a decent backup mage. 2 decent healers (both will level slowly, but eventually will get very powerful). A decent, but not great thief. Just to give you an idea of what, I expect, will be a pretty strong party in IA.