I make my own biodiesel. Have been doing it for about 3 years. It was a rough learning curve at first, but now it takes me about 2 hours of my time to produce a tank full of fuel. All at a cost of about .65 cents a gallon. I use cooking oil that I get for free from local resturaunts. It's not that tough to do. Their are some safety issues that you need to pay close attention to, but, just common sense stuff.
Some states allow you to produce a certian amount before you are held liable for paying taxes on the fuel. Of course I stay under that amount each year. I use the fuel in a Dodge Cummins and a Passat diesel. They love the stuff. Especially the Dodge (the Passat is already pretty quite). My truck sounds like a whole different vehicle. Less noise, no smoke at all, smother running. Bio is actually better for your engine. It has higher lubrication qualities than regular diesel, and acts like a cleaner to your entire fuel system. In fact, older vehicles may need to change a fuel filter or two the first time they start usiing it, as it cleans the fuel tank, the lines, the injection pump, the injectors, and the combustion chamber.
If you buy a biodiesel processor, it will be very pricey, and probably not any better than a homemade unit (sometimes the homemade unit is better). If you can make a few simple wiring connections, and can connect plumbing pipes together, then you have it licked. A simple and very effective processor is made from a new or used electric hot water heater. If you do a search on google regarding "appleseed processor", you have a months worth of reading material. Their are simple instructions out there, and it is not tough to do.
The big issue is a reliable source of good used cooking oil. That is the key to low cost fuel. Here is some simple math for you. It costs me $100 bucks to fill my truck (35 gal tank) with diesel. $22.00 for home brew biodiesel. My truck runs better on it, and if anyone cares about the environment, CO emissions are reduced by 75% (I don't care about that, I do it to save $$). Plus it is pretty much carbon nuetral.
No modifications are needed to vehicles that are made form about 1992 on. However, some of the newer diesels will be limited to about 50% bio because of the higher pressures in the newer common rail fuel systems. They tend to polymerize the bio, and can lead to filter clogging when running it in higher concentrations. I run 100% all summer long. No issues with my vehicles what so ever. I do take care to produce good clean fuel, and my process is a little longer than usual, but it gives a higher quality fuel, with very little added costs.
There are allot of good forums out there that are dedicated to biodiesel home brewers. Hope this helps, KD.