Much as I am a fan of Audi, the new Audis I have had (2 old shape A6s) both started to go wrong at around 70000 miles when only 4 years old. For me an old A8 would cost too much in repairs and servicing at that kind of mileage. Much as I loath to say it, perhaps an old shape 520d would be better? Great fuel consumption and it would be a lot newer with BM warranty still on it. The other option could be an A6 2.0tdi - once again a great car for fuel consumption and still with audi warranty on it. For 20k you should be able to get both at around 18 months old (if not younger) as they are either just replaced, or about to be replaced.

This one might do with some haggling:
http://www.audi.co.uk/used-cars/used...cmdc=se_na_re_

Jon.