Me personally; if I were interested in a '17 GTI or any "sporty" type of vehicle, I would have an unbiased mechanic do a thorough diagnostic on it before committing and even a compression test; to include inspection. People will do something like that for $100 / depending on their proximity to its location.
60K miles is already at or due for:
Transmission Fluid change (not to mention the condition of it)
Brakes (Discs & Rotors & Fluid)
Spark Plug Interval #2
Tire Wear
Engine Health (Cyl 1-4 Compression? Is the exhaust riddled with Black soot or stains?
Turbo Health
A/C
Alternator
Carbon (soot) on the Intake Manifold slides / Valves
Condition of the Cooling Stack? Is the A/C Condenser riddled with dents and bugs?
Is it "lowered" / How does the Composite Oil Pan look? Is it scuffed?
Etc, etc.......
If the owner or current business has MAINTENANCE records or maintenance proof by way of receipts etc, that may solidify things.
Only 1 Owner would be another sign only dealing with 1 previous owner.
But, did that owner TUNE that vehicle? If you really want to know, just look at the OBDII port and if it is scuffed, it was tuned bottom line - no &'s ifs or buts. Unless! They went with a JB1/JB4 - let the mechanic determine that.
So many things to consider.
In GER I don't know what price someone would pay for that '17 GTI, but in the U.S. I wouldn't pay more than $15.5K for an SE model. But, you didn't specify the model or trans type - nor from a dealer or private seller.
If it is from a Private Seller and they are promoting "Aftermarket parts or modifications" to the car, just walk away period. VW's are not cheap to fix and much of the time a lot of used vehicles were never maintained very well.
AND
When you do buy, never purchase at a price that is above what the Market Value would yield for the condition the vehicle is in - granted you do decide to purchase.