When I was converting D&D 3.5e spells, I used INT min for wizard spells, EGO min for Cleric spells, and PRE min for Bard & Sorcerer spells. Since I wasn't using a magic roll when converting the spells, there was no benefit for being over the min, and if you were under the min, you couldn't cast the spell. I used the levels for STR min, but since Characteristic Drains were quite rare in what I was designing, there could be an argument for a lower Limitation value.