All Audi diesels are capable of 200k+ miles so don't even give it a second thought. Provided they are maintained properly ie: regular oil changes, you won't have any trouble. They come in two formats, the 1.9 or the 2.5 - depends on what sort of driving you do but the 1.9 is more than capable even in a big car.

IMO there is NO risk.

The Audi will hold a stronger residual value as well. The C5 will depreciate faster than Prescot's fuel budget

We have the A4 and it's excellent