Does anyone have any links for design, parts sources at a discount, etc.?

I am planning a flat bed utility trailer with 2 3500lb. axles. I want to use tubing rather than channel for the structure to have a stronger/lighter trailer. Planning on torsion axles, led lighting, etc. Size will be 16'x7'ish, enough to allow for the fenders and tires. I was planning on using 2x4 .120 wall tubing for the structure. Questions I would have include proper design of the frame, weight distribution, axle placement (fore and aft), etc.

Any sources, links or suggestions would be appreciated.