Hi Lancalad, welcome to the forum!
I bought an ex-demo 24 plate Mk8 GTI back in April and have had absolutely no issues whatsoever. I have to admit I was a tad wary after reading about some of the issues on here, but that turned out to be unfounded. I have owned various GTIs over the years (I'm 74 now!) and this is one of the best in my opinion.
The Mk 8.5 was only just coming to the UK then, and as it turns out there are very few noticeable differences between the two as previously posted; OK, there is a slight power hike, but I doubt you would really notice during everyday fun driving. (I previously had a Porsche Macan S with 350 BHP, but the GTI is nearly 1/2 tonne lighter so the performance is almost identical).
As for the Mk8.5 interior, I just don't like the new infotainment screen - the Mk 8 one is much better integrated. Again, a matter of opinion, but I'm glad I took the 8.
Good luck with your GTI, you'll love it!