Turbo hoses often leak and really need to be replaced to cure it. The hoses under each side of the car just behind the fog lights where they go into the intercooler may be covered in oil gunk and crap - sign of a worn seal or connection and will need replacing.
Rad fans - with aircon on both fans should turn at half speed (ish). If faulty they either don't work at all or ramp up to full speed for around 5 seconds then drop and they do this periodically - a sign of internal resistor being knackered. The fans should run at full speed if the engine is hot but it won't be for 5 secs - something quite a bit longer than that! To fix it needs new fans or a homemade external mod as found on here somewhere! Dealer will charge around £400 to replace as they can't be fixed. Common across a number of VW group cars.
The electronic clutch on the aircon compressor can fail or the spindle that it "grips" can wear so it can't grip. New compressor from Seat is about £1k before fitting. Easy check - aircon on you will hear the clutch kick in with a strong click and the triangle centre will spin. If its not on, the central triangle bit will be static. The triangle bit is clearly visible as the centre of the "pulley" wheel on the compressor. The outer "wheel" will spin regardless as its driven by the belt.
Worn clutch - check for slippage by accelerating in 5th from 1500rpm and watch that the revs rise smoothly. On the PD150 the clutch is a weak point especially on remapped cars. You can also get a knocking from the dual mass flywheel (DMF). Clutch is about £125, the Flywheel about £250 plus £150 ish for fitting. You would also do the release bearing at the same time (£40 part).
Loads on all of this (other than the fogs and the shocks) on here as well known "faults" but they don't make these cars a bad choice and not every one has these issues - although the fans failing is very common as are the turbo hoses. The rest are not as common. I've had mine from new and covered 80k and other than wear and tear and the clutch (mine is remapped!) I've had no proper issues. The aircon did play up but that eventually turned out to be wiring damaged by the bloke who fitted the clutch!
The last weakness will be the urge to mod your car and drain your wallet that way!!!! Its addictive, and worthwhile.
