I've been reading over Curse of Strahd with an idea of running it at some point, and while there are a lot of things I like about it, I find Strahd himself... off.

Nothing about his backstory suggests he should be a wizard; in life he was a warrior, and he became a vampire by making a deal with dark powers, not study. And, well, nothing about my image of vampires involves them throwing fireballs. Back when Castle Ravenloft was first published the idea of a monster with class levels was pretty radical, but that was a long time ago.

The simple option is to change his spells around, and maybe turn him into a warlock (more consistent with his backstory), but part of me is tempted to build a staged boss fight of some sort, where he starts with basic vampire stats, but when he's struck down he reforms as something else. Any thoughts or suggestions?

I agree that Strahd seems like he should be maybe a Hexblade or maybe a Fiend/Blade warlock. But remember, since any experienced players may have also already read that entire module, it may be wise either way to tweak Strahd with a bot o' the ole'  homebrew just to keep the players on their toes.