As DrE wrote - there have been issues with the early s-tronic gearboxes on the B8 platform, but hopefully you'll be fine with a 2014 or later model. However if it has done more than 38K miles then make sure it has had its gearbox service done.
Again there have been historical problems with the B8 S4/S5, such as window motors, central locking, water pump, to name a few, but later models seem to have had these resolved. A 2014+ S5 will have the CREC engine and there are two issues with it. Both are in the higher £hundreds to fix.
1) The supercharger clutch can slip, however this will likely cause a CEL during a "lively" test drive.
2) The OEM crank pulley can fail - but likely only at higher mileage, say 80K miles plus.
I have just been hit by what could be a "known" B8 problem - my offside headlight washer jet was leaking.
I'm not familiar with 2.0TFSI problems, but again as DrE wrote there were serious oil consumption problems on these cars for a range of years. If I were considering buying one I'd probably do a bit of research to see if any owners of 2014+ cars have complained of high oil consumption.
