I can't offer hands on advice on the RS5 specifically but I would suggest that you need access to a mechanic who has experience with RS versions. I have two earlier RS and very few really know the tricks nor have the special tools. Luckily I have both myself so I haven't had to find someone. Even some Audi mechanics can be a bit perplexed with older cars, their primary reason for being is servicing newish cars and hardly anyone takes an older Audi to the dealer for work, so the younger mechanics may never have seen some models.
I would suggest that part of the purchasing process is actually finding this mechanical unicorn in your area that you could use. Rarely is there such a design issue (as opposed to general condition) with a car that you would walk away from a purchase but like all of the top end cars, keeping them on the road is not necessarily without some ongoing expense.
I don't recall any really glaring issues with the RS5 from owners on here but we will all have our own perspective on whats acceptable. Good luck.
C5 RS6: Sportec, Milltek & Wagner -- B5 RS4 : modified -- B5 Audi 1.8 quattro 132 Kw -- 2007 VW Touareg TDI 3.0 with air suspension and lockers-- UNIMOG 404 TLF8