Personally I would forget about what engine it is in the factor of deciding.. they can be changed and upgraded. a van thats in good condition and what you want from it are the important bits.. I would rather buy a nice one thats petrol than a scruffy one just coz its TD..
1.9 petrol or TD are about the same performance wise.. I believe.. TD will be better on fuel though.. 2.1 petrols are faster and about same mpg as 1.9.. petrols 17-25mpg, td 25-35 mpg
History and maintenacne are the key to all things mechanical, but remember the vehicle is at least 15 years old so dont expect it not to need money spending on it.....
Remember you can fit LPG to the petrol so can basicaly half the fuel costs if you do enough miles to warrant it..
Oh I have a 1.9 Syncro Doka and a Caravelle that was a 1.9 and now has a 2.1 running the 1.9 carb etc.. both were on LPG.. (Dont ask!!!)