The Mk7 / 7.5 GTI (2015–2021) is, for most people, the sweet spot — the EA888 Gen 3 fixed the worst of the earlier faults, the chassis is superb, and they're plentiful.
Mk7 checks:
- Water pump/thermostat — still a plastic module that leaks.
- Wastegate-actuator rattle at light throttle — usually just noise.
- Carbon — Gen 3 is better, but a high-mileage DI car still benefits from a walnut blast.
- DSG service; the VAQ diff service on Performance-Pack cars.
- I check for a tune — a lot are chipped; a clean Stage 1 is fine, a hammered one less so.
So the Mk7 is the reliable modern GTI to buy — service the water pump and DSG and it's cheap to run.