I'm at nearly 5 years of ownership on my 2016 [MY17] Turbo... couldn't recommend one more!
Air suspension is a must have IMO, especially if you're going to tow with it (I tow with mine, so ordered it with factory hitch)... I am with you on the wheel diameter choice, I knew I would be doing lots of roads trips and some off-road work, so stuck with the 20" wheels (most turbo builds opt the 21's) I don't think going down to a 19' wheel is a bad move either, but the 20's are perfectly fine blend of comfort and performance.
If buying used, I think its worth looking for the MY17 and up as they got the new PCM 4.0 and more standard equipment, also find one with warranty as that is super valuable if buying pre-owned.
Common issues in my experience with the given vintage (both 3.0 & 3.6L) are:
Timing case seals leak - there is a fix, dealer does this remedial under warranty
Diff bushes - the wear prematurely (again warranty)
Transfer case - more specifically centre diff can be an issue on low mile around town driven cars (my dad had one done on his GTS, as did another member here)
Those are really the big ones ^
As for options, that is so personal.. but the big ones for me: Air Suspension, factory hitch, ventilated + heated seats
Here is my love letter to the Porsche Macan!