Welcome mate,
I have a mk3 and the faults that I've heard of is rust around the windscreen and the bottom of the door's, apparently they rust on the inside of the doors (where the hinges are). They have a common abs fault which mine has and I've only just noticed so not sure of a fix yet? I've just fixed an oil leak that was my cam cover gasket and oil pressure switch. I've heard the throttle body sticks on some so it takes a while for the rev's to drop or they can stick. The electric window's have a comon fault but I think it's an easy fix and the drivers seat bolster is prone to bad wear but you will find them in good nick, if you do buy 1 with with a bad seat you can get an interior their 10 a penny.
That's all I know of, I'm sure you'll get more information of people with more knowledge than me, I've only had mine for just over a month so I'm probably gonna come across more at some point.
Good luck with the search and once again welcome
