Clerics rock because they can heal. This holds true at any level.
Wizards, like this Illusionist, should probably memorize Sleep as many times as they can. That spell doesn't scale at all well, (making it a bad choice for a sorceror), but is killer for the first few levels. Charm person is another good choice. Also, if the group does more than just combat, they might find the Illusionist's illusions to be useful. Minor Image and Change Self are both good spells.
With a Sorceror, I would just take pot shots with a crossbow, and save your magic missiles for when they are needed. Use them against boss monsters with a high AC. Use them against weak or weakened enemies that a guaranteed to be killed in one shot by the 1d4+1. Use them against monsters like shadows that can only be hurt by magic, since the party is low on magic items.
Also, at low levels, a wizard/sorceror with good dex and Mage Armor and Shield can usually only be hit on a natural 20. Although they might drop from one hit, I've seen them stand up to foes that would have felled the other party members almost certainly in a round or two.