Yeah the C7/7.5 platform is solid and very reliable. I have 160K on my 2012 A6, and the only major problems so far knock on wood, were the well known engine mounts. Looking to maybe move to an 18 S6 next year, and have had some of the same reservations, after hearing about the $10K repair bills for dealer intalled OEM turbo replacement. After talking with a few folks, though it seems that you can get aftermarket turbos installed by an independent for like $3K. So that made me feel a lot better knowing that worse case they do go, its not crazy expensive. ALso the stories about the Mechatronic unit going on the DSGs were my other concern, but that issue seems way less likely.