There are a few things to look out for
Cam cover oil leak - they used black sealer instead of a gasket. Common issue to leak!
Weak chassis battery. Original was 45ah and went flat quick if not used regularly, new version is 65ah. There was a recall on these.
Condensation in rear light clusters, very common issue. Mine has it!
Scratches on drivers door glass, caused by the window mechanism. Rare fault but mine has it.
Here is a link to the GT86 club, should take you to a thread for “what to look out for when buying a GT86/BRZ. https://www.gt86.org.uk/forums/forums/topic/5689-what-to-look-for-when-buying-a-used-gt86brz/
I can’t think of anything I don’t like, there are lots of modifications you can do. I’d like to go for more power potentially, forced induction with either a turbo or supercharger is the way to go. Up to 350hp is deemed the limit on stock intervals. But makes for a fast car when weight is circa 1250kg.
I raced a N/A auto supra at Santa Pod and it’s very close, I’d say the GT86 is a touch quicker though due to less weight. It’s worth bearing in mind that from the factory Toyota/Subaru quote 200bhp, they never make that on a dyno. They range between 175-190bhp!