My old D1 gave me 100k with no real bother , a failed swivel bearing caused its only trip on the back of recovery truck, to fair it had been clonking for the week previous , apart from duff batteries always started even in coldest of winters, ( I live high in hills of the Cotswolds winter can be harsh here)
It's had ; a brake calliper, oil cooler pipes, then service items like cam belts and brakes
A couple of road accidents that it shrugged off , ( one where I destroyed a panel van , peeled it open from front door to rear light)
So in my experience the reliability has never been a issue , just regular maintenance and sympathetic driving
Rust has been a problem , the UK fondness for using salt in winter has ravaged the sills and boot floor and that's what forced me to retire her (with 180k on the clock ) pending a marathon welding session when I get time .
I replaced her with another 300tdi , a late model ex jap import with no rust at all !