If you're out of warranty and you get German car problems.... God help ya.
I had enough of my BM Trouble You some years ago. Don't get me wrong, they're great cars when they function as they do when brand new. I would always consider renting them or leasing them, just can't own another ever again.
I'd say yes and no to that. My wife has her second Audi. First was a 2014 A4. Currently in a 2016 Q3. Both bought used with no warranties. Both high mileage. Never really had a problem with either of them. The A4 was just too low of a car for her to get in and out of multiple times a day, so she bought a Q3. Taller seat, so easier to enter/exit. Both are 2.0T engines. The A4 was FWD with the CVT.... and despite my distrust of CVTs in general, that car did very well for us. With the Q3, I can always smell a slight hint of coolant when she parks in the garage, but beyond that the car has yet to actually leak a drop. I've found zero seepage and no coolant loss in the tank.
I can't speak to specific knowledge on the S6 or S7 models, but there are a number of sources out there you can follow. Obviously lots of youtube content is out there. I'd also spend some time on FCP Euro's youtube channel. Not just Audi, but other brands they carry parts for, they have videos on basic service, parts replacements and things you should watch as an owner. I can't recommend them enough for learning.
Audis are just "higher class" Volkswagens on many levels. But I've been very pleased with our ownership experiences of both Audis.
Although my attention is now directed at a Lexus LS430 that my wife acquired from her parents' estate.... I spend very little time in her Audi now. Given the option, I'd love to have a RS6 Avant. Happy trails on your journey.