We made test batches and ran them in a diesel truck- the guy that owns the truck SWEARS the thing runs better (and it might- 100% biodiesel tends to clean out the fuel system (and clog the fuel filter in the process)).
After a few batches we quickly realized that we had a problem- glycerin. When you make biodiesel, you end up making a LOT of glycerin.... and this glycerin contains some methanol, so you don't exactly want to flush it down the drain, or make hand soap out of it. I did have an idea for solving this problem though- there are TONS of apple orchards out here, and all of them have 'smudge pots' they fire up in the fall to protect the apples from frost. I was going to mix up some of the glycerin with kerosene or something to see if it could be used as smudge pot fuel. Heck, I would GIVE the crap away if people would use it. I just haven't had the free time to investigate this yet.
The second (much less minor) problem is taxes. If/when we get to this point, we might just dye the biodiesel red and sell it to farmers.... this way we wouldn't have to collect and deal with any of those headaches.
If your DH wants to save money, he could directly burn the waste oil from your local Chinese buffet.... no conversion to biodiesel needed. It's trickier to do (he would need to modify the fuel system on the Mercedes), but he wouldn't have to deal with the biodiesel manufacturing process. Google for greasel conversion and you'll see what's involved. For the right person, it could be a really fun project.
