You've already mentioned the two npc's I think of as must-haves.
You'll need a thief of course. The possibilities are as follows:
A decent halfling fighter thief. He comes with Xzar who you could either use in place of Edwin or push off a cliff. Both Montaron and Xzar are referenced in BGII.
Shar-Teel can dual class to a thief and makes an awesome backstabber. Her drawbacks are she's so good at backstabbing, you'll be tempted to put most her points in stealth and neglect spot traps, and then you'll still need another thief, plus you have to weight if you want to level her as a fighter first. While she doesn't appear in BG2, there is a minor plot twist involving her in BG1.
He comes late in the game, and is another mage-thief where you already have two spell casters. Still, he does make an appearance in BGII.
Pretty much an Imoen replacement. You could use her until Shar-Teel duals.
Available only late in the game.
You'll also want some muscle (in addition to Shar-Teel if you use her) and Kagain is great for that. He doesn't appear in BGII, but I can't think of anyone else so would still recommend him.
So perhaps the PC, Edwin, Viconia, Shar-Teel, Kagain, and one other thief type?