Mk7 and Mk7.5 are the same car, just a few tweaks to power and aesthetics between models then later models 2017+ you have additional spec in the form of heated seats as standard and 2019+ a rear camera, then 2017/18 saw the introduction of the GPF filter to the exhaust so if you like changing exhausts you should factor that in.
Must haves would be performance pack imo as it helps retain value, with the bigger brakes, power bump and Limit slip differential.
Things to avoid:
Rear door speakers on 5 doors can leak and let water in, seems less common with the facelift models.
Rear bumper vents can leak and let water in, also seems less common with the facelift models.
Sunroof's can crack around the glass surround.
Bonnet cable housing can come apart, so if you buy just add a cable tie to it.
Otherwise there's not too many issues with them, and if you're unsure go for a VW All-in-One service plan or approved used as you get 2 years warranty.
I'd go with a better example of a mk7 over a mk7.5 with a budget of £20k, prices are still inflated which doesn't help.
