I have a local steel/fabricator shop I have been dealing w/for a few yrs now. If they don't have what I need they can order it for me, and I have it in a day or two. The shop does a lot of fab work for race cars, farms, and trailers, general repairs, so they have a variety of material available. A large steel distributor should have all the gauges available, if not check w/a local fabricator. It will also be cheaper to buy drop steel(scrap) then to buy complete lengths of steel. It takes a little longer to gather the pieces you need from the crap pile, but it can save you a lot of money.

OOP's first design could still be built rather lightly. I would use either .095 or .125 for the outer bottom edge, and .065 for the inner rail(the one that sits above the frame rails) with a ladder design, and diamond plate steel covering the ladder design. Something similar to this.

This way any impact to the outer rail is distributed equally along the entire outer and inner pieces, reducing the force of the impact. Then 1x1 for cross bracing between the inner two rails(main bed section) and either cover it w/expanded metal or a piece of diamond plate aluminum. The aluminum will be expensive, I had considered using it but at $6/sq.ft. it become rather expensive really quick. A 6'x4' piece would be $198, and most shops will charge you for the entire 8' piece, so figure another $48.

For the top rail you could use either .095 or .125 1.5"x1.5" sq. tube, it should be strong enough to withstand trail damage with the same type of ladder design.

Once you figure out how much steel you need say if you went with all 2x2, you can figure out the weight with this chart.
, then see how much weight you can save by using different size/gauge material where you don't need 2x2 or a heavy gauge.
And rectangle.