Hi.
Still a newbie with all this, and UK based , but I have built my own , a KISS style , a small military, box style trailer, using new and second user steel, no problem to fit rails to attach a RTT, though not required in my case.
Link-http://www.expeditionportal.com/forum/threads/64134-First-Trailer-Build
With basic leaf spring suspension, a box on wheels to match my Land Rover Defender., I accept leaf springs and a beam axle have limitation, but cost came into to the equation, and the suspension set-up would be capable to cope with any terrain I would encounter in the UK.
I based my simple build on this post on another forum.
Link-
http://www.wildyoats.com/off_road_trailer_build.htm
Impressed with what people have done with adapting a small trailer from such companies as “Harbour Freight”, as all they need is a trailer that can be used off road, and they will not be taking the truck / trailer over extremes of terrain., and can use the trailer for a trip to the dump or general use., assuming you do not need it outfitted with every thing including the kitchen sink
Link-
http://www.expeditionportal.com/forum/threads/70824-My-little-Harbor-Freight-trailer-build
There are other posts on a similar vein, as you have probable seen. The first question you must ask yourself is “what do I want from the trailer”, a basic trailer, with offroad capabilities, or do you want all the bells and whistles including the kitchen sink, shower and full catering facilities ,or happy with basic leaf sprung suspension to just carry your kit ,or want to fit air bags and independent suspension , a massive choice and difference in cost.