Check rust in the sunroof runners, a common place for it to start. also in the door/boot hinges, they're another favourite.
i think the ssame applies for every other car, look out for km's, paint defects or damage etc.
Oh, but make sure it has the front diff/axels if you're getting an RS4. someone in NZ bought what they thought was an RS4, but turned out to be an RS (2wd)