All robes in skyrim are clothing, which have no defense by definition. Only light and heavy armor can have a defense rating. In your case I wouldn't bother with smithing since it doesn't sound like you're really gonna be wearing armor or even using weapons all that much. I'd probably go with enchanting myself since you can easily buy or find most potions you really need and as a mage having good enchants on your robes makes a huge difference, especially if you're not gonna exploit the alchemy/enchanting loop (which I personally wouldn't suggest doing).