You don't necessarily have to invest in Magicka for your thief, just make sure to get the right perks and train Illusion whenever possible. The only spells that require more than 100 Magicka for a decently skilled and perked character are the Master ones, and I get the feeling your character wouldn't need to use them often. Furthermore, spells like Invisibility and Muffle only need to be cast once every few minutes, giving your Magicka plenty of time to recharge.
Thing is, illusion becomes obsolete unless you use higher-level spells

