The 1.9tdi is probably one of the most solid mass production engines ever built.
Been a VAG group gem of an engine used throughout their entire range, all the way from 90hp to 150hp, from a3's, a4's, a6's, golf mk3. golf mk4, bora, passat, vento, polo, octavia, fabia. They are popular because they are good.
They will do 250k+ if given regular oil changes.
Had a mk3 golf 90hp 1.9tdi with 100k on the clock when bought, owned for 4years, without a single problem with the engine. Sold with 150k on the clock.
My old mans A4 90hp 1.9tdi has 190k on the clock, owned from new, not a single problem with the engine, still going strong.
Bought an A3 110hp 1.9tdi with 180k on the clock the engine pulled cleanly, still going strong. Although everything else around it was shot.
Its a cracking engine you cant really go wrong with.
An example with ~130-140k on the clock with pretty good service history should make you proud for many years without any problem. With that kind of milage the engine will be absolutely no issue, its everything else around it you need to check. If your anal about it, all suspension/steering rubbers/bushes are likely to be "going off", shocks likely to be also, cambelt, waterpump, powersteering pump, thermostat, wheel bearings, clutch. The usual suspects.
I personally would love an A3 PD130 1.9tdi Quattro Sport, but they still demand ridiculous quantities of money.
Good luck with the purchase, and enjoy the derv MPG.