Hi!
My last car was a GM engined Mk4 1.7.
I looked into the veg oil thing at the time, and I'm sure that there is a pump issue, or I would have done it. I'm not sure but I think at the time I found out that there are two different pumps on this model one will take veg, one won't
Also, (unrelated) you need to watch these for the fuel pump failure, as the pumps are quite expensive, and built into one of the engine mounts (so a pig of a job).
Only problems I had with mine over about 50K was the waterpump disintegrated (luckily the cambelt stayed put) and the EGR valve solenoid was duff from the moment I bought it (and I never bothered replacing), although it did go into limp mode when the EML came on, I figured out that it would reset at a certain rev range.
Also took it to a place called van aaken (brillian diesel tuner down south sadly now bankrupt) and they put a tuning box on it taking it from 75bhp to 95bhp which really made it fly! (Although I know you're not bothered about that side of things).