You don't really need to go caster heavy in BG1 but I tried it and it's one of the most fun groups I've run though.
My group is:
Imoen : DC to Mage at level 5
Using this, this let me put a lot of situational spells I don't often use on my Sorcerer to get more use of out those, plus he's a MM cannon with the Ring of Wizardry.
Dynaheir uses Web of course and lots of offensive spells.
Imoen has self buffs and de-buffs, like Greater Maliasion, etc. and Ring of Free Action and the Belt of the Antipode with other Cold Resist as well for > 100% resist.
Minsc uses Spider bane.
It's such a powerful team for BG1. With wands on everyone, you can walk into a room and Fireball + Fireball + Skull Trap or whatever, etc. Solo characters don't stand a chance.
On harder fights it's Web and Ice Storm with Minsc wading into the Ice and getting healed from it!
Before Spider Bane Minsc used the Composite Longbow +1 on Web fights.
Really powerful group!