There is a difference in pickup body trucks and flat bed/ service body trucks.
If a truck came from the factory with a pickup bed, the frame will be slightly wider and curved behind the cab. If the truck came from the factory as a cab and chassis, the frame will be a bit narrower and straight behind the cab. I don't think either one is really better than the other, but be sure you know which one you're looking at when you start pricing beds. If you want to put a pickup bed on it, make sure it's not a cab and chassis truck. Most cab and chassis trucks are designed for at least a 9' bed, and with the frame being a slightly different width it takes a lot of work to put a regular bed on one. It's possible, but it's pretty labor intensive.
There's a local flatbed place that sells an 8' by 9' steel flatbed for cab and chassis trucks for 1,090 or so. Not sure what the specs on it are, or if that includes install. Just seen their sign. They usually have some pickup beds sitting in the lot.