"He asked me how thick I would want the bed and if I wanted to use diamond plate?"
Are you referring to the thickness of the decking material, or from the frame to the top of the deck?
If your intent is to only haul a TC, the thickness of the decking material can be pretty thin. The TC will span the cross braces, which support the weight of the TC. Fact is, to haul a TC you really do not need a deck. If you haul things other than a TC, then you have to consider the strength as it to pertains to loading the stuff you haul. With a real thin decking material (steel), it will dent easily. You can't go to far wrong with a wood deck. It will not dent, stuff does not slide as easy as a metal (steel) deck, and if it gets damaged it is easier to repair. I have had Steel, Aluminum (1/4"thk), and wood decks. Aluminum is my first choice, but cost a lot, wood is my second choice.
If the thickness you refer to is from the frame to the top of the deck, you then must decide if you want the deck flat, or with short wheel wells. Flat will raise your TC, whereas wheel wells will lower the TC. I have had both types for hauling TCs.
Wayne