About four years ago I owned a 1998 Audi A6 2.5 V6 Tdi 150ps with 130k miles on the clock; this is presumably the same engine fitted to the A4 that you are interested in.
Common problem with these is gearbox pinion wear (after about 100k miles) that causes a slight high-pitched whine on over-run (mine had it but it never caused a problem). Big job to fix but the gearbox could continue to run 'forever' with the problem.
My A6 leaked oil from sump. Not a straightforward case of changing the sump gasket - not likely! The V6 has a 'split sump' that requires removal of the oil pump (amongst other things) to replace the gasket(s).
Apart from that it was a nice drive, went like a rocket and produced very reasonable MPG when plodding along.
I'd have another (much newer) one if given the choice.
Hope this helps.
Cheers - Shedpainter
