Which used Audi A6 saloon should I buy?
The diesels are best: they are far more common than the petrols, and hold their value better. The entry-level 138bhp 2.0-litre TDI performs well in such a large car. If you want more performance, then the smooth 178bhp 2.7-litre V6 is the next best option, with the 233bhp 3.0-litre V6 diesel the quickest, but also the rarest.
Consider a petrol model, though, if the price is right. The turbocharged 2.0-litre has 165bhp, then there are three V6s and two V8s with between 177 and 345bhp. The 2.0 and smallest V6 are the best options.