Well on my old MK4 Golf GTI 1.8T 20v I sold it with 145k on the clock!! The engine was excellent and pulled very well. (it was also re-mapped by upsolute). The engines can last a long time as long as serviced well.
Only thing to watch is regular oil changes, make sure cambelt is done on time and the waterpump has been changed with the cambelt as the plastic OEM part is prone to fail (plastic brittle part). I also had to replace my oil pump at 130k miles.
All in all I have never had a issue with the 1.8 20v T block.
Loads of info on the site in regards to servicing, buying guides etc