Take Tucker's advice, you're going to max out quick with a 350 I can speak from experience. Carlos beat this into my head which I didn't take the time to actually listen to him but the 350 is the exact same size and exact same cab/chassis as the 450 and the 550 difference is suspension and brakes etc and gearing for the engine. I can put about 8,000 pounds of wood/chips into my 550 so no sweating a dot inspection and the curb weight is probably close to slightly more than same year 350. I don't think price is a major factor on older used trucks.
Also, find a 7.3L truck if you're going ford, or plan on spending several thousand to fix the problems with the 6.0/6.4L diesels. If you find a good deal on a 6.0 which there are a lot of them out there factor in several thousand to get the bugs worked out.
jp