I would look for a REALLY good deal on an A4 '04. Information I've received is they are in very low demand from dealers- - -so if you can find one, I think it would be at a very good price.
I haven't checked KBB or Edmunds, but I would try to buy one for $15-$17 thousand. Back in Dec. '04, with incentives from GM and GM Card, you could buy a brand new one for ~ $20K