What about a spellthief as a base. Seems like a natural fit and a way to "charge up" nicely as well...
I'll have to read up on Spellfire a bit more, as it's been a long while (was the inspiration for the custom tattoo on Kell's left arm that became so powerful it was hereditary), but I wonder how Master Spellthief would interact with that.
Spellfire does not make you an arcane caster as its a Supernatural ability so Master Spellthief doesn't help. This is part of the reason why i've avoided spellthief. You've gotta get in close to get your power which is just too dangerous and time consuming...especially for the power you're actually getting.
