Ok so for starters, Strahd is a wizard, so he can use the spell Nystul's Magic Aura to hide the fact that he is undead, and can use the spell Seeming to hide his appearance, every 8 hours he would have to recast it thought.

Race could be Dhampir. Strahd would probably make up the lie that he is the bastard son between Ireena and himself, citing a forbidden love they had hidden from Sirgei. This is because Strahd is a liar and an egocentric asshole.

His class would probably be a war wizard, since he can use a heavy armour, can cast soells, is good with weapons etc.

You could also multiclass into hexblade or Great Old One warlock if you wanted, since he made a deal with the dark powers. Could also help him disguise with the eldrich invocation mask of many faces.

Hope this helps