[mad=dog]

I don't think Assasin->Sorceror and Kensai->Sorceror is valid. Solo characters always need a lot of juice. Prepare to roll for some time.
Strength - 18 (essential):
You need a lot of strength because you need to haul everything by yourself. If you are not playing BGT you may face problems with the non-warrior types because you cannot get 19. Rely on belts then. Ass->Mage cannot use Crom Faer either.
Dexterity - 18 (essential):
Especially for the kensai types who will have to cope with very high AC. Contributes to missile TH and thieves skills as well where applicable.
Constitution - 15/16/18 (essential):
15 if you play BGT and non-warrior. 16 if you play SoA and non-warrior. 18 if you play warrior.
Intelligence - 3/18 (essential to some):
Only essential to mage duals. Very essential in that case.
Wisdom - 3 (redundant):
Something has to go. In this case Wisdom. Make sure to purchase a few potions of insight and drink them when you cast Wish-type spells.
Charisma - 3 (redundant):
4 words. Ring of Human Influence. Just slip it on in non-combat zones (which usually happen to be negotiation zones) and you will be dandy.

To put it in plain language - sacrifice Charisma first and Wisdom second. Don't accept less than optimum in everything else.