How slow of a spellcasting progression should a versatile at will caster get? [3.5]

[Temotei]

Probably...4th - 6th. I'd go with the lower numbers if you post it on here. It's hard to say because of the wildly varying powers of spells in that range. Past that, though, spells start to completely destroy reality, defecating on its remains. Those spells aren't really an option.

Granted, enervation at will would be absolutely awesome and probably overpowered.

Half-casting spell lists do get spells earlier than a lot of full-casting classes, but that can be fixed by raising the spell level by one--maybe two, if you're really insecure about it unbalancing the game. I don't think you are, considering you're going for an at-will caster.