Things to look for buying A B8.5 S4

I am upgrading to a 13+ S4 with my VW TDI blood money, and am starting to look at examples. What S4 specific problems should I keep an eye out for? Also, I have read differing opinions on the sports diffs reliability (obviously reliability always changes with tuning but I am referring to more catastrophic failures) with tuning. Are these warranted or just a vocal minority?