One thing to consider is the hours of engine use. Many commercial service trucks have low mileage, but have high hours, because they run the engine all day on the job, powering winches, booms, compressors and other job-related equipment. There is usually a procedure to check the hours of newer diesel trucks. You would have to consult the owner's manual, or ask on a brand-specific forum, to find out how to check the hours on a truck you're interested in.
Some trucks with flatbeds or service beds started out life originally as pickups and had the beds replaced. Others, started out as incomplete cab/chassis trucks, which were then sent to an aftermarket upfitter and finished as a complete vehicle, with the bed and other equipment.
Depending on your state's laws, the cab/chassis truck may be considered a commercial truck, requiring commercial truck registration and insurance, or it may not. In AZ and CA, cab/chassis trucks over 10,000 GVWR are automatically commercial trucks, for registration and insurance purposes, regardless of how they are used.
I had two F350's, otherwise nearly identical, except one was a pickup and one was a cab/chassis truck. The cab/chassis truck was roughly double the cost on registration and insurance, versus the pickup, both in AZ and CA.