I pulled out the little 6.4L Powerstroke Design Guide I got when I bought my truck. "The use of biodiesel is acceptable as long as the rating does not exceed 5% (B5)". The guide also says that if biodiesel is used, that the severe duty maintenance schedule is required (oil change every 5,000 miles).
I've never run biodiesel in my truck but I'm sure others on this site can help you out. The only thing I would worry about is the source of the biodiesel and the quality control.
The 6.4L is proving to be a very reliable motor if well maintained. Ask for the service records and have a good mechanic take a look at the truck before you buy it.