Sounds like in the beginning even the sc could not diagnose the issue and it caused the original owner some frustration, hence the idea of selling it. Feels like a good buy. Vw cars are more common and down to earth than its luxurious siblings (Porsche, Audi, .....) . If more experienced guys , they would have bought this car from the owner and check system one by one. Usually just small part to replace/ repair.

Usually would have started with obd, then check if any sensors having issue. Sometimes sensors are at their end of life they might work 50% of the time, hence undetected during service. I also would have checked battery, alternator. But less than 40k on the odo seem to be a good one. Let me know if he still think about selling.