The RNS510 with DAB and navigation will just plug and play (unless youhave Dynaudio installed then you'll need to do a bit of coding on the head unit so it doesn't blow the amp up!) and you can pick them up for a few hundred quid. Plus, because it's OEM it's more appealing when you sell the car. Also, don't worry about getting the latest revision, few people on the mk5 Golf GTI forum have reported there's little difference at all between the revisions apart from the middle period ones having LED backlit screens which look nicer. Save some money and get a mid life revision (H-P should be good if I'm correct).
The Kenwood DNX525DAB is a direct factory upgrade available direct from VW but as mentioned, pick on up off eBay and fit yourself to save hundreds of pounds. Again, the unit supports all OE features but has the advantage of being able to work with full audio upgrades (external amps and subwoofers ect.) but if you're not going to go nuts with the speakers then you're likely be better of getting an RNS510 unit.