The ECS rotors last a lot longer than the OEM rotors for a street-driven car. JHM makes a slightly less expensive set that are just as good. A pad swap, right off the bat, will also help with the wear issue. The other option is to find a car with optional ceramic brakes. They'll last just about forever if you're not tracking the car. Also, the fronts are the ones that tend to wear out quickly, not the rears. Some don't have any issues with front rotor wear.
If you're mechanical, the cost of maintenance drops substantially. Oil changes are around $100 for DIY, full synthetic with filter. It's totally doable to swap pads and rotors yourself although you'll need a VCDS for the rear brakes to reset them. Audi says you can go 10K for oil changes, most people do half that as previously mentioned. If you are a DIY kind of owner, plan on purchasing a few new tools as German cars use a lot of faster types not found on Japanese cars.
Carbon buildup is the biggest "issue" really. An independent shop can do a clean for about $800. Figure doing it every 35 to 40K miles to keep things in tip top shape. Cars go longer without issue.
Servicing is critical on the RS5. Make sure the car has documented service at the appropriate times. You didn't say how many miles the car had but there's a 15K, 30K, 45K and 55K service. The 15K, 30K and the 55K are pretty important and you need to make sure the S-tronic and rear differential were serviced on "both sides" at the appropriate mileage. The gearbox and rear differential have a side filled with ATF and a side filled with gear oil. One or the other or both are changed depending on which service it is. If I remember correctly, the 30K and 55K are the most expensive. You can also purchase Audicare in advance of the service and save some money. The 45K service is a glorified oil change.
A lot of owners purchased Audicare ahead of time. It stays with the car. So you may have one or two services that haven't been used yet. They do expire and your Audi dealer can tell you when they need to be used by. For example, I have one Audicare left, the 45K service, which expires in April. I'll have my car serviced in March before it reaches 45K miles (unless I
really drive a lot here in the next month). So it's mileage
and time based.
Tires are going to be more expensive but there are affordable options out there that are competent.
Overall, as others have said, the car is pretty darn reliable. It's no E90 series M3. Those cars are complete nightmares.
On what to look for, make sure the car shifts smoothly, especially when driven slow. There shouldn't be any big shudders or clunks when coming to a stop or starting out from a stop. Make sure the rear spoiler deploys and retracts correctly. Make sure the gas cap flap opens and closes correctly. Turn the wheel fully left and right at a stop and turn slowly and make sure there are no clicks. The front differential will "skip" a bit but you're coming from an STi so you'll be familiar with the idiosyncrasies of AWD cars.
It's also not unheard of for a suspension bushing to go bad on Audis.
The rest of "what to look for" is standard used car stuff.
Misano Red is the best and fastest color. Fact. I'm completely unbiased.
As others have said, it's not the fastest. I've heard people call the RS5 "big boned" but honestly, the car doesn't feel it. It hides it's weight well and it'll have serious, serious pace if driven how it should be driven. My biggest complaint is you have to be going darned fast to have a lot of fun with the car. It can be frustrating, daily driven, as windows of exploitation are few and far between. But it's so real-world capable, any weather, any time. It's a truly great car with a dual personality.
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