One thing I would check if going for this deal is how much less it would be without maintenance. 20k miles won't require much in regards to tyres and servicing unless you drive like a tit everywhere. 1 set of tyres and an oil and filter change will be around £750 if you shop right.
I sincerly trust you would change the oil more frequently than 20k miles on a Golf R!!! 
When you've finished with it, that said Golf R will surely be bought by an enthusiast who will more than likely visit forums like this asking for a detailed history . . . . 
I personally would only service it when it requires a service and no sooner than that. My last 6 cars have all been leased and all been serviced when the service intervals require it. People who want to service their cars every 5-10k are more than welcome, but I honestly don't see the point tbh. If they want to spend their hard earned on servicing their car more often than needed on the hope that an 'enthusiast' buys said car in the future and keep him happy, then fair play to them
My cars have always been warmed up correctly and had everything they have needed with no expense spared. However, they only get serviced when they require it.
All have done between 80-110k in 3 years and none have ever had any mechanical failures or unhealthy running issues. That all being said - my cars spend a lot of their life on the motorway and rack up the miles pretty quickly. So if it were to be a car that did 5k a year, then it would be a completely different story and they would be serviced once a year.
