I'm no expert on what is a major fault for DSG, and what can be cured with a simple reset, so I'll leave that for others.
Car revving up & down on idle is usually a sign of a failed PCV. It's a very easy DIY fix, and the part is around £35.
If the car doesn't perform as well as expected for a 200bhp car, it's likely the DV has failed. Again, not a difficult fix but a bit fiddly on a normal GTI.
See here:
http://www.mk5golfgti.co.uk/forum/index.php/topic,44709.0.htmlThe cam follower should be replaced when you buy the car, part is around £35 and there are hundreds of DIY threads, or take it to a local specialist if you don't fancy trying it. A 2008 car shouldn't have the awkward banjo bolt so shouldn't be too hard to do.
Cam chains are typically noisy on AXX engines (2004-2005). A 2008 model with the BWA engine shouldnt suffer - especially not with such low mileage.
All that ^^ can be found on the forum, so try a search if you are desperate for a quick answer in future