Trying to read between the lines here, somewhat, of what the OP's needs are but maybe a conventional quality built trailer and a custom suspension would meet the goal.
I had posted about converting an enclosed utility trailer, previously, and the thought there was that many of them come with a fairly rugged suspension. Since they are designed as a hauler of weight, the frame and suspension have to meet that goal. As Profdan states, there is considerable design and implementation time/expense involved. It is a project that most would not take on.
This is why my current thought is that some fabrication of a better suspension and possible frame upgrade may be a better avenue to get what appears to be that "niche-filler" that is sought. I believe more rugged axles and springs, all set up similar to the what Aussie trailers have, is not going to empty the bank account. My experience with frame shops is that the work involved costs less than expected.
If the OP only desires an on-road trailer that can endure some rugged use and environment, the nearly all-aluminum trailers that are available fit that use admirably.
