Hello welcome mate.
Now where to start, the 1.8 Turbo and the 150 TDi are the pick of the bunch but not by any means the cheapest. Depends what you want to spend.
The 1.4 is very under powered but ok for around town.
The 2.0 8v drink oil for a pass time and are only 115 bhp, not great as the mk4 is a heavy car.
Also depends what you want from it TBH. Thay can suffer from some common probs,
the windows dropping into the doors is very common tho most mk4 have had this issue sorted by vw as it was such a big prob. The 1.8T can suffer from faulty coilpacks.
Above all the mk4 is very smooth car to drive and normally very reliable.
Ive done 55000 miles in my 1.8t and its been as spot on.
HTH

rich.