In one of these posts, welding was mentioned as a means of construction. I would really try to avoid welding and see if you can have all the connections designed for thru bolting with gusset plates. I would really worry about the welds (or material around the welds) cracking over time. I think in the end you will have a trailer that lasts longer if you avoid the welding.

Otherwise, I like the design and have actually drawn up something very similar. Just haven't had a chance to get it built yet...

TB