1) As long as they're priced correctly I would say buy on condition rather than age and mileage. But if £500 gets you a car in the same condition, service history etc i'd pay it if the mileage is much lower. Don't just check bodywork, interior etc, but also things like how much life the tyres have in them. GTi is chain driven rather than cambelt. GTi manual clutches aren't great, so test drive and look out for signs of slippage, high bite etc.
2) On mk7.5 you can alter the cabin noise via the Soundaktor to eco and it will be quieter inside. On Mk7 the only way to turn the engine noise down is either by putting car into eco mode or with vagcom / obd11.
3) Sunroof is down to personal taste. I don't care for them so wouldn't pay extra to have one. It's just something else that can go wrong in my opinion.