BlueRidge,
The rig in that pic isn't mine, just something that caught my eye. What you do depends on if you have a trailer already or not.
If you have one and it is leaf-sprung then I would stay with leaves. Just go with longer ones and add mounts for dampers. The rear's off of a late 90's GM pick-up are 63" long, about the longest easily available leaf out there. Might be able to score some for free to cheap from a shop that installs lift kits. Then it's just a matter of cutting off the old mounts and welding on the new ones. WFO Concepts & Ruff Stuff are the two that come immediately to mind as sources for the mounts & related bits. I got the parts to put these springs under the rear of my '84 Yota Xcab from WFO. You may or may not need to remove a leaf or two to get the rate agreeable. On the Mini-truck I removed the two shortest leaves. The shorter the leaf length, the stiffer it is so start with removing the shortest first.
A long time ago an employee of Deaver Spring, mfg of desert racing and other uses leaf springs, told me to set the shackle hanger so that the shackle points to about "4:00" More vertical than that and the ride is stiff, more laid over than that and the ride is too soft.
If you don't have a trailer and plan to buy one I'd buy it with the torsion axles and specifically ask for the trailer brakes with the parking brake function. It will likely be an extra cost option, but will pay off when you park it unattached to the tow rig in a camp spot. Come to think of it, I'd add this to an existing trailer too. Then have the mounts for dampers added.
It should be noted that torsion axles come in a couple different ride height indexes. I'm not sure of exactly what they are so research this before buying.
If you are going to design one and have someone build it who doesn't normally do that sort of work I strongly suggest M.M. Smith's "How to Design and Build Trailers, vol. II". It is out of print and getting expensive but is worth it. I've not found an in-print book that is as good. Volume I is up to you, personally I didn't get much out of it. Unfortunately they are self-published, so check Amazon. This book goes over every aspect of designing a trailer so that it works as planned and does so without bad behavior.
These are how the lower damper mounts were added to the torsion axle under our Rubicon Trail ready camp trailer: