If you're trying to strictly follow Dresden's progressing narration, then I see one of two routes:
The Apprentice. You're learning your first element (ie Harry learning Fire from Justin), and you have Channeling. As he gets older, he learns more elements (upgrade to Evocation). Storywise, this makes a lot of sense w/ game progression.
Set in My Ways. If you're playing an older character, it's a reasonable assumption that they are "set" in their ways, and cannot learn more diverse magic (like trying to learn an additional language when you're older is harder). THIS IS COMPLETELY GM/PLAYER's DISCRETION.
Basically, look at how far the game is going to progress, and in what direction, and if the player may want to diversify, and decide if "Set in My Ways" fits your campaign. If it doesn't, drop it. Your Game, Your Rules.
(as for the defense, basically what others have said- Atheletics to dodge, endurance to brush off the hit, counterspell if applicable)