Brill. In that case the 2.0tdi is your best bet. If you're looking at an auto make sure the gearbox oil has been changed and if quattro make sure all tyres have even tread on the same axles. As uneven tyre wear can damage the quattro system.
With the age of car you're looking at electrical issues are the biggest thing. Make sure AC works as a new compressor can be very expensive to have done.
Also make sure the car comes with 2 keys that work fully and the central locking works.
Main rust spots are the tops of the arches on the front wings as VAG thought it was clever to put foam in the inside of the top arches on all vw/audi models like the golf mk4, A3, A4 b5, A6 c5 etc.
I'd also have look at buyers guides on YouTube.