I believe all the sunroofs are aftermarket, mine is a Hollandia (sp?), and if I didnt have the paperwork on it I would swear it was factory. As for other things, I think it would be wise to ask about would be, has it had window track problems, and have they been fixed. Also if you are tall, consider the height issue, I am 6ft and if the previous owner hadnt lowered the seat I dont know if I could have adjusted to it or not.
Other than that, the obvious research and mechanic check.