you two make me laugh...
anyhow, to the matter at hand - i own a 2.0T and i do agree with aythreee that surely the 2.0T is the natural progression from the 1.8T, but as it's a 2.0TDI you're after you should look out for the following:
early cars had problems with cylinder heads cracking. if you seach on here there's a thread that tells you where to look for the part number. cylinder heads ending in "A" had the most problems, "B" had a few problems and "C" had no reported problems. if you're going for a car newer than 2007ish you should be fine.
there have been some reports of turbo failure as well, due to weak internals, but i gather this is more of a problem when remapped.
if you're on a budget then Sport is fine, but i'd recommend an S-Line.
FWD vs. quattro is a separate debate, but i think it's down to personal preference in the 2.0TDIs. PilotAudi overstates the virtues of quattro i feel, and having driven a 2.0TDI 170 i feel FWD is perfectly adequate. if you're tuning it tho, then quattro all the way.
the final question is DSG or no DSG - for me it's a no brainer. DSG is phenomenal and i'd never go back to a manual but you can't have quattro and DSG together in the diesels
generic A3 things to look out for if buying 2nd hand are to push all the buttons and make sure everything works as it should. i've owned 2 A3 8Ps now and have had problems with everything from the parking sensors to the remote locking, so have a good nose around.
most things can be retrofitted, but the most complicated to do so are auto lights/wipers (requires a new windshield) and if you're wanting to retrofit nav try look for a car with double DIN head unit. swapping the cage is possible but annoying and you'd need a new climate control unit as well.
the only thing left to ask is if you're buying new have you considered a Golf GTD? it's cheaper and (in my opinion) a much better car