On top of my head this is what I look out for:
- Check the service history of the car (if it has one at all), the MK5 GTI can run either the standard time and distance servicing (1 year/10,000 miles) or the longlife regime (2 years/18,000 miles) but the latter should be avoided for these sort of cars. Also check whether the cambelt has been changed (4 years or about 60k) as it can be a potential engine breaker if it's not done. Should the car have no history it's most likely been neglected so it's best to walk away.
- Check the MOT history of the car as any advisories could be something that you will need to sort out at some point. You can do this by using the governments website, plus it's a good way to check any mileage discreprencies too.
- DSG gearboxes needs to be serviced every 40k, check that this has been done. Also while driving does the DSG feel jerky/hesitant when changing gears? Signs could be the mechatronics is on the way out which is very costly to repair (1k). Manual boxes are generally rock solid, should the shifter feels notchy it probably just need a gear oil change or the linkages reset.
- Have a listen to the engine, does it sound like a diesel? If so then then the cam chain tensioner will need adjusting, not a cheap job this as it's normally around 700-800 from garages as it's quite a big job.
- The OEM DV (diverter valve) can fail which means a leak in boost and reduced performance, this is because the diaphragm is made out of rubber and can rip. You can either replace it with a new DV (Revision G) but have the same problem later on, use the plastic piston design (Revision D) which eliminates the rubber problem (but doesn't hold boost at high loads compared to the rubber versions) or the aftermarket GFB DV+ which solves both problems of the OEM version (about £98 so double the cost but it's a one time fit so cheaper in long run).
- Check the Cam Follower is in good condition near the HPFP, it's like a tiny thimble and can wear out over time. You need to catch this before it fails completely as it can be quite catastrophic to both the car and your wallet if it goes. Typically it needs a change every 20k to be on the safe side (maybe sooner depending on tune) and it costs £30 which is a lot better than paying thousands fixing a camshaft failure.
- Check the front arches, bubbly rust is very common on this generation Golf. Some (not all) VW dealers will honour the 12 year bodywork warranty and will fix this free of charge.
- Even though I own a 2005 model I would say go for at least the 2006 plates upwards. The earlier models (04-05) are prone to whiteworming on the rear badge which looks nasty and the interior handles have a rubber coating that will scratch very easily, they are replaceable though so not all is lost but it's just an additional expense. This was rectified on 55 plate cars onwards using different part numbers. The later models (2007) have a revised engine (BWA) to beef up the reliability from the previous AXX engines. The BYD units fitted on the Edition 30 are even stronger and are tunable beyond 300 BHP with little work.
- Check the interior condition, particularly the bolsters on the front seats as they are prone to wear and even rip which doesn't look nice. Can be costly to fix/replace the seats so it's better that you don't have to with a well looked after example.
