I have had mine for 6.5 years and 80k, I wouldn't buy one if you are looking for cheap motoring.
Mine needed the four rear suspension arms replaced due to seized adjusters @ 70k which was needed to allow the wheel alignment to be adjusted (they are only £35-40 each from VW so cheaper than paying to get all the bushes in them replaced). I decided as the front end was feeling a little soft to do a full suspension refresh the difference was night and day I was surprised how soft the old bushes were, the top mounts were also very soft compared to the new ones.
The MK5 Golf also has a known fault with the front wings rusting from the inside out, VW will replace these under the anti corrision warranty, I needed two new front wings and I caught mine very early. I also had small rust spots on the sills that had chipped the paint off the bottom of the doors causing a couple of spots of rust on the door. Fairly minor but VW picked up the bill to spray the doors and sills so I paid to get the rear 1/4s, bonnet and front bumper sprayed so my car ended up with a near total respray at 75k (Looks brand new now
) If you look at the front wings of MK5 Golfs you will be surprised how many have rust, if you run you finger round the edge of the arch it should be smooth.
Engine wise mine has been fine, I have replaced the cam follower twice now which runs the high pressure fuel pump. Sometimes these are known to fail and take the pump and cam with it the part is cheap at £25 and easy to change. Timing belts/watar pump are recommended to be changed every 40k or 4 years, so going by this most of the early GTIs should be on the 3rd belt however many haven't had any changes. Dump valves are known to go but cheap to replace I got mine from Seat for £30 and changed it over myself.
Brakes last well, my rear pads needed replaced around 50k the fronts at 70k ish.
I had a rear bearing go around 73k which started with the ABS light coming on with a rear sensor fault, no noise was being generated. I expect the other side to go shortly as this also happened around the same mileage with my sister's A3.
The 18" alloys corrode and are hard to get properly refurbished, I got a new set under warranty from VW (2K !) just as the warranty was ending. I look after them and they are starting to go again. I am going to get them refurbed at Lepsons after the winter is over which is aorund £400-500 if you can't drop them off and pick them up.
The stock headlights are poor so you ideally want to get one with Xenons. (not a huge issue or expensive to retro fit as they come up on ebay fairly regulary).
Pretty much every option is easy to retro fit, I think the RNS510 sat nav unit is superb and couldn't do without this including the highline cluster and MFSW (I retro fitted all of these) The MK6 reversing camera thats pops out the rear badge is also handy.
All in all in the last year it has cost me a little over 3k but this did include four tyres and a major service. I do like to keep the car in good condition and could have saved some cash on the respray and suspension work by only changing the bits that needed done to get the wheel alignment done but in my mind if one bit is going the rest won't be far behind and I was very happy with the results of a full refresh.
EDIT Just remebered that mine had a new DMF, VW replaced it under goodwill for free. I couldn't tell it was gone they only noticed when they had the gearbox off due to a noisey release bearing which they also replaced for free. I had to pay for a new clutch other wise the old one was going back in think it was £120 ish. (The old clutch had little wear at 50k) This seems an isolated case from what I have seen on forums but I would expect any clutch change to require a new DMF at the same time.
