I haven't driven an R36, but the obvious difference will be the naturally aspirated R36 vs the turbo induction of the Arteon. On paper they seem to run similar times. The Arteon has a bit of boost lag, but even a stage 1 tune makes a large difference to performance. EA888 engines don't mind running higher boost and the DSG gearbox will keep up without any issues.

Being a larger car I was impressed how the Arteon handled stock. I had expected I would need at least upgraded sway bars, but it sits surprisingly well through corners.

If the car was not dealer serviced, keep an eye out it had the S-Suction-jet pump recall completed, you can put the VIN in here:
Service Campaigns and Recalls | Volkswagen Australia

The centre touch screen for a few VW's around this age had an issue with black lines running vertically down the screen. Some threads here say they may replace them outside of warranty, but worth checking with the owner.

Also worth noting that 2019 Arteon's will have the 5 year warranty, rather than 3 years.

MY17 and early MY18 also had traction control (ESC) on/off as a button next to the gear stick, but was removed in later models. It can still be toggled via the touch screen but not as easily accessible if that is important to you.