Angle and c-channel are pretty common in commercial trailers. Relative to square (or rectangular) tube - it's cheap & light. It also isn't as durable. Depending on what you do with the trailer - that may or may not be a problem. Stop by a trailer sales place and look around. Look under the commercial trailers and see how they're put together. If nothing else, it will give you an idea for a starting point.
When I built my motorcycle/utility trailer, I used 2 x 4 x .120" HREW tube. 2 x 4 is a pretty convenient size to work with. Spring hangers, lights, etc all fit without building extra tabs and mounts, saving that headache and keeping the lines clean. .120 (1/8") is thick enough that when mounting tie-down loops (etc), thread cutting screws have enough material to work with, eliminating the need to weld them on.
Up-size the axle, downsize the springs. Use shocks & as big of a tire as practical.
For a total load of about 1200 pounds (2 motorcycles, gear, + the trailer itself), I used 1250# springs & a 3500# axle, 205/75 tires (IIRC).
Be careful with weight - if you make it too light relative to the spring rate, the suspension won't work. 1250 pound spring were the lightest I could find locally. With the trailer unloaded (about 500 pounds), it bounces all over the place, despite running near 20psi in the tires. It needs a little weight on it to make the springs work. If the suspension doesn't work - it transfers all that energy into the frame & box, which isn't a good thing.
"Life is meant to be enjoyed, not endured" ~someone