Welcome 
1. Me personally, I would never buy a demo. I would buy second hand, but not a demo as it will have been thrashed.
2. Yes, for sure! 10-12% is easily obtainable if you're willing to spend a few hours in a dealership!
3. I recently bought from Citygate in High Wycombe - ask for Alister - he was excellent, and I would defintely buy from him again. Also, my original order before I sourced a car quicker was with Rosh at Vindis in Sawston (Cambridge) - he also was excellent. Both those dealerships were first class in terms of keeping me updated, finance stuff etc etc. Ones to avoid (in my experience) are Lancaster in Letchworth (Herts) and Inchcape in Romford - I won't explain my reasons why but suffice to say I won't be giving them any business again in the future!
Bought a demo 2.0 Sport VW Bora many years ago with 9k miles and we ran it to nearly 85k miles with only one failed water pump (common on that model) but the oil consumption was ridiculous!! and my thinking (rightly or wrongly) was that this was due to having less care in early miles and from cold starts (IE: thrashed) than privately owned.
Not sure if any of that applies to the GTI/GTD in potentially occurring though?
Would buy a car from garage if an exchange though but would want to see service history to see if not exchanged due to someone giving up on it due to niggles.
My GTD is nearly at 10k with first service at 12k and having checked oil just recently its not lost a drop!
