Another option if you must have a thief is a fighter/mage/thief. Fewer HP, but an elf can get to level 11, and you can cast Hold Monster, Haste, and all the buffs.

You can probably keep most of the rest of the party. Some might swap out the single-class mage for a ranger and dual him or her to a mage, but it's a matter of choice.

it depends how min/max you want to go really.

i think the "best" party, if you can stand the xp slog is 4 dual classed paladin/mages (takes forever to roll them right if you don't use modify), a dual class cleric/mage and a dual class cleric/thief. paladins are the best class when you can't rest, mages are the best when you can.

getting the paladins an extra attack at level 13 is pretty nice, but stopping at a 1/2 attack is okay, it just depends on how long you want to spend preparing to play the game vs actually playing it. if you go into the 4th game, that can completely change you're ideal dual level too, again depending on how min/max you want to go.

that said, SotSB is a VERY easy game, especially compared to curse (beholder fights, bit o moanders, margoyle/tyranthraxus etc), so beating it with your existing party is possible, i would lose the elves though.

also, dualing into mages means you'll need to keep bracers of AC in your inventory when you want to cast, so be aware of that. and your thief can use scrolls (75% success) once he's level 10, so technically, everyone's a mage.