I would defintely look at CoC if you want some really detailed insanity rules.
Note that in a standard DnD world, many kinds of insanity are curable via spells like Restoration and its bigger brothers.
If you want to go with ability damage, Wis is defintely the way to go with insanity. If I remember correctly, one of the MM monsters (the Allip) has a touch attack that is supposed to drive the target insane. This effect is represented by 1d4 Wisdom drain. The real problem with using ability damage this way is that it leaves the insanity portion completely up to whoever is playing the character.
It would be theoretically possible to continue to play the character exactly as before (although this would be bad role-playing.) CoC will give you a good number of real world mental disorders, and in-game ways of making sure they are played out at least somewhat.
The Maddness Clerical domain is a good way to have an insane Clerical spell caster that can actually cast spells. It probably works better than straight Wisdom drain, again if you want to go with the ability damage route.
As a final note, a powerful Cleric of a Neutral God turning his back on his God to become a Lich isn't automatically insane. Some Neutral Gods might not even care. If that is a story affect, thats all and good. Just making sure you weren't laboring under a false pretense.
