I've looked into this quite a bit over the past few months, and bought a MB diesel to do this project. The main purpose of shutting down on diesel is to prevent coking in the injectors which vegetable oil can cause. Veggie oil has actually proven a better lubricant than diesel in the engine as proven by higher compression on motors previously run on diesel and currently run on veg (go to website at the end of this post to see these data). The MB indirect injection engine is probably one of the best bio diesel motors out there. Pump is lubricated by hot engine oil that helps heat veg oil, veg oil is more viscous than diesel so needs to be "thinned" in order for your motor to run properly. You can do this by chemical conversion before putting it into your tank (deesterfication) or by heating it before it hits the combustion chamber. The MB also has those little spheres that the fuel hits against before combusting, this helps to atomize the mixture. Basically these motors were made to run on all kinds of fuel since they were sold all over the world, and the quality of diesel varies quite a bit depending on where you are....go to eastern europe sometime and check out the air quality from all the "dirty diesel" sold there.
for lots of good info. check out this site....
http://www.biofuels.ca
they sell the kits, but more importantly the guy that designed the system is very helpful and knowlegeable. The subject of his masters thesis (which is available online) is running biodiesel in a VW rabbit. hope this helps.