Everyone including myself will tell you they go forever if you buy a genuine and maintained motor. My old 1.9 105 Passat did 15k a year with me and reached 185k. Still rattle free and drove beautifully, I'd still have it if it carried sheet timber. The problem is with Leon/golf/jetta they're all prime for working vehicles due to their reliability so expect to pay a premium for anything under 100k. Personally I would aim for a well serviced full history vehicle at around 120k, check your belts and clutch as they're usually ready around that mileage.
The other thing with VAG diesels is they seem to drop into limp mode occasionally if they're pinned hard, my polo used to be a swine for it which turned out to be the air flow meter was just shitted up. Replaced it with a Bosch item at £60 and it never happened again.