Timing belt/waterpump/tensioner & Suspension wear - Control arms seem to be the bigger cost maintenance problems.
PCV plumbing failures, coils, mafs, O2 sensors, coolant sensors and occasionally the crank position sensor will all fail on higher mileage 1.8ts. I came from a much lighter MK4 1.8t so I think the mileage in the B6 kinda sucks because it weighs so much more. Mine's gonna stay bone stock for some time, but it was nice having AWD over the winter. My BT GTI gets 27mpg on average with plenty of highway romping while the B6 gets 24mpg on average.
Good luck finding a MT, Avant, 1.8t locally. I gave up and bought a TIP sedan nearby. If you're planning to add stuff like a bigger turbo and go that route I'd look more at the B5s personally.