I'd say anything over 150k on a mk5 tdi is creeping up into higher mileage territory but that said I'd still happily spend my own money on one with the mileage.
When you go to view one that slightly up on the miles just make sure there's service history there and it's been regularly done then just check things like steering wheel wear, seatbelt wear, gearknob, pedals, seats etc
I'd say average mileage per year in a diesel car should be atleast 15k so 150k on a 10 year old model is bang on average so that's an easy way to work out if it's high mileage or not