Playfield graphics are the big one to look out for, sure. There are some reproduction playfields out there, so even that can be fixed on some titles, but doing a full playfield swap has to be a "labor of love" and not just something you feel like you have to do. It sucks. But you know your machine inside and out, too.
Most anything can be fixed, and all the parts for all the generic assemblies (flippers, pop bumpers, jet bumpers, etc) are available, but occasionally you find a game-specific playfield part that is NOT available any longer.
--Donnie