OK, let's go from the GTD forwards.
The SB and RA motors are OK and nice and top-endy, especially for a diesel. 90BHP, but all at the top of the rev range.
They are distinctly more muscular than the standard 1.6TD, but suffer the same weaknesses - the fibre head gasket and also the heads, which spit out the metal between the valves and then crack and let water into the combustion chambers and allow gases to pressurise the coolant.
FAIL.
The AAZ is a nice motor, also surprisingly free-revving, but not in standard form. It's also not intercooled in standard form. Get a cooler on there and put in the boost enrichment pin from an SB or RA motor, turn up the pump and you're rocking on +100bhp and more torque.
Then there's the early electronic TDI - the 1Z. nice little motor, but not very strong and not as revvy as the indirect injection motors listed above.
Then you're into the 90/110bhp electronic TDIs. More sensors, more power, stronger in AFN guise (IIRC the code correctly)
Then on to the PD motors. More complex, more tuneable, newer.
Bear in mind that you have two distinctly different strains of these motors from the 90/110 onwards - the transverse motors from the Golf platform and the longitudinal ones from the Passat. These will have quite different manifolds and mounts, as well as very differently configured ancilliaries. I would strongly recommend you get something from a Golf platform motor to avoid huge ball-ache.
On the 90/110 motors you also have the option (much beloved of the T25 brigade) of fitting a mechanical TDI pump from Bernd Jaeger in Germany. Dig deep if you're afraid of wires and rejoice in 120bhp.
If I was to go for a 'quick' conversion, it'd be a 1Z out of a Golf3. if I wanted big power it'd be the PD 130 motor, tuned, or running on this:
http://www.bodylogicuk.com/90265/info.php?p=8Although I wouldn't ever put a TDI motor into a Golf2 as a transplant.
