Don't be put off if it doesn't have a charge or shows a random economy figure. Picked up my 2018 estate in December with no charge. Just make sure the cables are with it. I only got the granny charger as I was unaware at the time that it should have come with both.
Had mine for nearly 4 years from new (2017) and the only slight issue I've had is fogging of both headlights when raining. This only started to happen after about 3 years and only does it intermittently. No idea how you'd check headlights for this issue other than testing it in the rain! VW said the fogging wasn't substantial enough to replace the units under warranty.
Very minor issue in what has been an otherwise faultless car.
Our GTE estate was very reliable as well, only problem I had was an intermittent infotainment screen that sometimes just stayed black, replaced under warranty. I'd go with the usual checks that all ancillaries work, condition/wheels/tyres acceptable, fluid levels are properly maintained and ensure service history is up to date (inc DSG gearbox oil change at 40k mile intervals). A really great PHEV IMHO.
I would say pull over to some parking or something and try to crab on very slowly (engage and disengage the clutch with the brake pedal if you know what I mean). See if there is any sort of vibration or noise or sharp kicking. So just before you stop release the brake and repeat. This will let you test the Mechatronic unit as far as I know to see if it works correctly. The used GTE I bought from October 2016 has had its Mechatronic unit replaced in 2020 under warranty.