For my own paying clients, I have something in there that says if they put someone on my servers, they are telling me that they control the copyright for the item and if they don't, they take full responsability over it.
I get 4-5 copyright complaints a month. They get dealt with right away without issue. (They're usually very obvious.) Only three have gone to court. One's pending but for all of them I was released from the suit.
Keep a good record of what you're doing, show that you do pay attention to copyright and complaints, and it shouldn't be an issue.
I agree though. You need to talk to a attorney. Even have one on retainer to bounce questions off of.