I'd avoid the 1.6, its quite a dull drive and not exactly economical, I'd also avoid the 3.2V6, not that its a bad engine, its fantastic but it costs a fortune to run!
With the years your looking at most will be 2.0 TDI, its a popular choice for a reason!
Also just as important is the spec, i'd avoid the base models (you can spot usually spot them with wheel trims and no fog lights. theya lso dont have DIS which is a must have in an audi!) At least get an SE or a Sport. The SE is more of a grown up "classier" car, it has wood and soft suspension. The sport rides slightly harder and comes with all the nice bits like sports seats, 3 spoke steering wheel etc. There are also the S line models that are based on the sport but have even harder suspension, 18 inch wheels and half leather. Then you also have the option of the DSG box....it gets quite complicated!!
TBH, unless the suspension is too harsh for you, I would at least opt for the sport model as the wood look interior is naff.
Some of the TDi engines have had problems with the cylinder head cracking, hopefully some of the other members know which engine it was. Steering racks have been know to fail prematurely, noticed by a creaking noise. Early model wiper motors have been known to fail. Some members have had problems with wheel bearings failing.
These problems aren't common though, but I guess they are better checked.
i got a 06 sportback s line full spec inc rnse satnav lightpack heated seats cruise control xenons in front.. bose sound in lava grey for 14850 pounds its an awesome car i love it also dont get the 1.6 its really slow i had one mine is a 2.0tfsi now and its cool really good engine sounds better than deisel and i was supprised how good it is on petrol almost as good as the 1.6 i kid you not that 16 was terrible but deisels sound like tranny vans also takes a while to take off petrol is much more responsiv id go petrol ans lose a little change on fuel but a much better ride