I've built four trailers, one for a boat, and it is not a job that I'd recommend as a saturday afternoon hoot. They were all built with new steel and required considerable tools and skills.

The idea of using the back of a pick-up/ute works, but it still needs a purpose-made tow bar on the trailer, designed for the body that it is going onto. The differential should be removed from axle to save needless friction. The brakes too, unless the intention is to link them up, which is not a job for the tyro.

Our local builders chain sells trailers at barely more than the cost of the materials. They are made in China, and not bad. I wouldn't build another trailer unless I needed something specialised, it isn't worth the effort. Unless the effort is the object of the exercise.