First of all, again I would like to highlight that this mod is amazing. It's the best time that I've had playing my favourite game of all time. It's amazingly balanced, works very well offers such a great fun developing an amazing and super powerful party. I've spent more hours on this than I care to count.
I posted a few notes when I started regarding my original party & composition and the tremendous troubles I had finishing, well, pretty much anything in the game. I started with dual classed Fighter/Thief, Cleric/Mage, Fighter/Druid with Barbarian and a Necro (PC). I had to make numerous alterations as this crew just wasn't going to make it.
Based on that incomplete run & my experience with the mod I took another swing at IA, this time being much wiser so my crew turned out to be: Necromancer (PC), Minsc, Valygar, Cernd, Jan Jansen and a custom made Riskbearer. I finished SoA with this crew but were in tremendous troubles by WK level 4 (especially the "orc"-encounter & Githyaki supreme leader). While it looks like a very powerful group, it sort of runs out of juice with some of the encounters, because it lacks pure melee-power.
So I made a few changes, re-imagined my approach to party composition (& took a few freedom's) but this is what I ended up with.
Protagonist: Vagrant (Inquisitor is the preferred & more powerful choice or at least for my playing style)
Sorcerer (I used Imoen)
Berserker(9)/Mage (We use Jan Jansen tweaked)
Kensal
Swashbuckler(5)/Cleric (We use Aerie for this, tweaked)
Berserker(9)/Druid (We use Jaheira, tweaked a bit)
^ This party is uber powerful. It has 4 outstanding kick-assers for melee with the possibility to add a 5th one in Swashbuckler/Cleric. We can get amazing amount of attacks per round, amazing Thac0's, we are extremely competent in spell casting having almost 4 full spell casters (2 arcane / 2 divine). If you want to have a thief in your party, this is probably "as strong as you can get". (Well, you could always make a Swashbuckler(3) -> Fighter but I like my cre as it is & I want to try a cleric).
My über-crew doesn't have a single IA kit. (or wouldn't if I play with the most powerful combination)
A note. I have an item pack installed giving me a few powerful items. I also bent the rules so in my game the S/C (turned to vampire) can use the cloak of the daywalker & she also got a powerful upgrade on her "symbol of deity" she got at level 25 (I think). She also can use thief specific gear, even if it's marked for "single class thief's only". By end of spellhold, we don't have single such item though and her usefulness just grows from there on.
Her "deity's holy symbol" gives her +2AC & Saves, +20HP, -2 Casting time & +10% physical resistance (but no +1 strength, no magic resistance, no extra spells) It's a powerful item, comparable perhaps to the "master of disciplines". While the casting time might seem like overkill, it doesn't come with "aura clensing" so she is limited to only 1 spell / rd . What it does do is make her low level spells even semi useful in combat because now they can be cast and sometimes even finish. I think this tweak works very well and isn't OP and makes Cleric more usable.
Jan (or Berserker / Mage) got an incarnated elven chain giving him -1 casting time, AC1 & 20% fire resistance. He also got a ring with an additional spell from levels 1-9. The ring is sort of optional, as I haven't run into a case where I would run out of spells with him. So, even though it's very potent I might just turn that item slot over to a ring of protection +2 later on.
For balance, we omitted a number of very powerful items from the game that would have been super useful incl. Jan's enhanced armour (it's very, very effective for a F/M), Githzeral Blade, (Enchanced) Armour of Stamina, Royal Elemental Staff, Noble Staff of Earth, Noble Staff of Air, Golem Slayer (it would be ridiculously effective with a berserker/druid), Sword of the Rashemi (we use Minsc for Kensai) & Might of the Avariel (Aerie is the T/C), Lord of the Underworld, Cloak of Arcane Immortality & Scarlet Ioun Stone, The Truth +5 and a few others.