TDI engines are generally very strong.
when test driving one make sure it pulls cleanly form around 1500rpm until at least 3500rpm (starts running out of power after 3000rpm) in all gearsÂÂÂ

test for clutch slip by putting it into 4th at 30-35mph and plant your foot; also do this for the other gears!
loads to look out for each with their own characteristics. all depends on your budget tbhÂÂÂ

the best and most popular ones are the
110
115
130
150
the latter 3 are the PD engines which offer more power and torque along with a 6 speed box.
http://www.golfgtiforum.co.uk/index.php?topic=38557.0that link will tell you what to look out for; sadly its a long list but don't let that put you off the MK4ÂÂÂ
