What's "mpg"? Put fuel in and enjoy - what else is there to care about?
- since you've stated that you're interested in a B8.5 it means that fortunately you shouldn't have to worry about some the problems that affected the B8, however the B8.5 isn't free of them.
The biggest issue is the s-tronic gearbox, as you may already know. I believe that the 3rd version of the DL501 was introduced around May 2013 and so if you can afford a late 2013 or newer car then that's the safest option. However the 2nd version is still a lot better than the 1st, but make sure that it has been properly serviced (ATF and filter) no later than every 38k miles. Since we have seen that cars have been falsely advertised as having had this service it will be worth seeking proof (i.e. an invoice). Irrespective of the car's age make sure that the gearchanges are smooth - if you sense any jerkiness or hear any noises then walk away.
Make sure the cooling system is OK. Early cars had pump & stat problems but IIRC @arad85
had a coolant issue on his.
Also check oil level. Although very rare, some of these engines consume oil - usually a PCV issue. Of course if the seller knows this it will be topped up.
There have been central locking & window problems on some cars so make sure everything works correctly.
A few people (including me) have been hit by leaking headlight washer jets. Test them and then look for leak (pull flap with a fingernail).
Longer term you'll want to do something with the crank pulley because enough of the OE ones have failed to class it as a known fault. This can usually be resolved when tuning.