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!!