It's a lot of food for thought, I'm still trying to get my head around buying new. I was raised never to buy new cars and that seed seems to be planted quite deeply.
For example there is a PP 5 door manual in white, registered July 2016 for 25k. It has nav pro, park assist, rear view camera, dynaudio and Santiago alloys. That, to me, is a good buy?
(Oh and 4000 miles on the clock) 
That being a "good price" depends very much on how much you value the extras. They add very little to the p/x price when you get rid.
Santiago alloys? A matter of taste, I personally find them a bit fussy, especially when you see the black spokes.
Dynaudio? Will you notice the difference when you're prodding the loud pedal and playing MP3s?
Rear View camera? Quite easy to do without as you have the sensors, and if you really want it, it is a cheap option on a new car.
Park Assist? For me a novelty item you'll use twice to impress your mates, the Golf is very easy to park - do you park on a street where you have to parallel park into a gap only 3ft longer than your car? If so, another cheap option on a new car.
Nav Pro - do you use the nav much? Would you pay £1300 for it on a new car to get a bigger screen and a bit better functionality?
It's about £340 (broker price) for the Park Pack that includes the camera and park assist. You've already stated an interest in white -a free colour, so going white and the park pack is almost paid for with the broker cost I quoted for Tornado red added in.
How long are you looking to keep the car? With the one you proposed, you've lost 5 months warranty - not a lot lost, but access to cheap service packs and cheaper APR % if you're reliant on VW finance is gone.
PP - probably the most value for money extra.
Not buying new makes perfect sense when buying many cars such as Focus (not RS), Astra, Megane etc that can be had for sometimes as little as half RRP at 2 years old, but for the prices used VWs are sold for at franchised dealers then it's usually not so cut and dry.
Take a 5dr PP in White, with parking pack (Park assist and camera) and Dynaudio through drivethedeal and it comes in at £24629. So you buy new and you lose the Nav Pro and half the cost of the 19" Santiagos vs the used one you have seen.
Would you rather have the extras or that new car smell, 5 months more warranty and access to cheap servicing for 2-5 years, as well as the option of buying a comprehensive 2 year extension to the 3 year warranty if you plan on keeping the car a long time?
For me, that used one isn't cheap enough next to the new one for the extra toys it gives. You will have to wait for the new one to be built of course!
You were talking about £21k cars a few days ago and now are up to £25k in your considerations - easily done!
