Are you buying the real wheels or replica? If it’s replica then you will need hubcentric rings. Note about hub rings, they are a wheel alignment tool that is left on the hub. They are a purely consumable item, metal ones are a waste and can easily get stuck. Get the hub size from the manufacture spec sheet and order some from your choice of retailer, often if you ask your retailer they will include them with the wheels. If you stick to the stick Golf R size the diameter should be very close to your stock wheels. Expect a difference in ride, the shorter the side wall the less your tire absorbs the energy from bumps. See what fits com will educate you on backspace and diameter anywhere from +42 to +50 is a very safe et for Golf chassis cars +35 is considered aggressive. Tire width will effect gas mileage the most. I dropped 4 mpg running 255/35/18 on my GTI lightly modded. Also weight of the wheel can effect gas mileage, which includes the tire. The bigger the wheel in the replica world typically means the heavier the wheel. So be careful what you buy and weigh your stock wheels or Google it, chances are if your doing it, somebody has already tried it and has weighed your stock wheels.
hope that answered your questions