Well, my '97 A3 is afflicted with the rusty doors sills, but I haven't noticed any corrosion around the roof seams (last time I look anyway!). If you're looking to buy, make sure to have a good look at the roof seams/gutters and the bottom edges of the doors, just above the black strip (on an A3) or the side skirt (on an S3). Another place worth checking is around the rear wheel arches. As with any car, also look out for overspray on rubber trim etc. - accident damage repair can make later corrosion more likely.