The Audi A4`s are cracking cars. If your after the diesel one, they are very economical. The early 1.9`s were a bit noisey, but he PD ones which came out in 2001 onwards were cracking. Nice and responsive, quiet, smooth and you could get into the 60mpg`s with them. When they facelifted the a4 they added the 2.0 diesel engine which is slightly different - same as in the mk5 golfs, but still good.
The a4`s hold their value well, and a lot of them are ex lease / company cars, so you`ll find a lot have 100k on the clock. But, thats nothing, they will just go and go. No major problems, just make sure the cambelt has been changed as per schedule.
I looked at quite a few of them a few years back and the only problem I has was there really were so many to choose from. In the end I went for a VW Golf instead as you got a lot higher spec for the same cash as an Audi.