I'm on may third A6 Avant with the 3Litre diesel. I'm approaching 100,000 in my current Biturbo. Absolutely no mechanical problems with any of
them. The ZF eight speed gearbox is well tried and tested and doesn't require the maintenance of the S-Tronic. Likewise, no cam belt to change.
Servicing is done by local garage or specialist . I average 37mpg in mixed driving.
Best avoid any cars with panoramic sunroof - I'm in the middle of the well documented leaky sunroof carry on.