With WotLK I would add as many classes you can handle. Warlock, mage, shammie as DPS, and Priest, Shammie as healers. 2 paladins as MT/OT/OH. I would have a healer that can be respecced to fill a DPS slot when you gear your healers so that you'll only need 2 of them. I would guess that you will need 3 healers in the gearing up period.

Which ever you decide, good luck!