Brimy, are the screws your talking about like the ones on snowmobile trailers? Filips head, drill tip, self taping threads? If so I would recomend not using them. My snowmobile trailers have them and they suck. Also, I would not make the trailer out of square tube (2x4) as you mentioned eariler, espically not using these screws. You will get water into the tubes, it will never get out and the trailer will rust out in 3-6 years of use if used in the winter at all, slightly longer if only used in the summer. Using a structurial "C" channel is a better way to go, it is as strong as square tube and you can paint it to protect it on all sides. Just a coupple FYI's from other trailers I have owned in my life... square tube on trailer though is bad unless all your welds are air tight and your not screwing into the tube to allow water in.
Also, something that makes it nice for running the wires is to just weld steel conduit to the trailer and pull the wires through that. After re-wiring several trailers I have started to do this on all my trailers. It makes the wires last MUCH longer.