I just bought one of those myself, though I'll probably keep him the same. I'm thinking about using a Proffesor X body...and a Caretaker from Ghost Rider head.
I found a few Black Masks from the Secret Files: Rogues Gallery Series 2 from DC Direct. So I think it'd be cheaper to buy that version and then turn him super articulated through sheer force of will.
No idea, I don't read comics as much anymore. I usually learn about characters by having people ask me for commissions, that's the way I learned about the Squadron Supreme
Red Skull would be good, but a Ghost Rider movie figure could also be used. Still don't know what would work for a base body though. Usually Black mas wears a basic dress shirt and tie right?