C8 models 2019 onwards. 2.0TDI engine is robust and will be paired with 7-speed S Tronic gearbox. That needs serviced every 38k miles. V6 variant, then 8-speed TipTronic.
Diesel 2.0 litre will be timing belt, so renew along with auxilliary belts every 60k miles plus water pump, etc.
Personally I’d look for Quattro. That adds a few mpg but it’s surefooted and will help sell thereafter.
Like anything, servicing is key. You want paperwork and lots of it. Avoid long service intervals. These are also sensor heavy, so consider purchasing VCDS as checking and clearing codes is £200 from Audi.
All diesels, then you’re into AdBlue (Euro6), EGR, DPF, etc. Not sure when mild hybrid system started on petrol and diesel, but it’s complex. Search VAG Technic. It might have been an A8 but it caused them grief to fix.
Faults, then rear coil spring seats are known weak point. The alloy corrodes from within the rubber. Rear subframes are just powder coated, so get there with Lanoguard. Front multi link suspension usually good for 100k miles, but will be hydrabushes, etc. On car around 50k miles, then potentially some arms if any knocks. Lemforder or Meyle HD.
Another option is A6 Allroad, that’s air suspension.
Extras worth getting, matrix headlights, auto towbar if you tow, full panoramic. It’s all money, but something to consider.