A member on this forum's truck I really like. I want to do something similar, but with wood as the actual bed and a metal frame work. Brazilian Walnut also known as IPE. I have some 5/4 boards that are 7' long. White Oak would be a good alternative, but is a little bit more than Red Oak. If you want to want to go the budget route...I'd buy some Pine or Douglas Fir 2x material and then mill it to 1 1/4" thick (5/4...5 quarter inches). Research different finishes and decide what you'd like to go with. I'm more of a carpenter than a mechanic, but I do what I can with my truck and can't wait for the bed!
oh yeah...the meatal. I may have line-x or a similar material sprayed on mine. We'll see what the budget is like at that time.