Hi Ken,

No major problems with that generation of A6, although you need to look for smoking engines (blown turbo) and scored or heavily worn rear brake discs (the electric parking brake isn't disengaging properly). Oh and for some reason the glove box lids are prone to sticking. 

As for mileage, you're quite right to be suspicious of a diesel car with low mileage and the 2.0 TDI does have a particulate filter so check the service record (and never buy one without a full service history) to see if the filter has had work in the past.