Well I've got a mk4 gt tdi - had it for 2 years, had a steering rack and rear caliper and your usual service parts.
Mine only does 30-35mpg urban and upto 46mpg on long run
It is a 115 chipped, by who I don't know. Fuel economy has gone down so I think the egr valve & inlet needs cleaning out.
Ride is a bit harsh but I have anniversary GTi suspension.

Check the rear wiper works fine as they pre 04 models has a bad design and the pipe unclips above the roof lining and pisses out down passenger c pillar and can cause rust
I think they are pretty strong but check the clutch