Looks like a cool project! We’re getting ready to help someone with some retro fitting of a military trailer. We always love talking to others and sharing our ideas. You should look us up, no charge for brainstorming ;-)
It will definitely be heavy but that doesn't have to be a show stopper. What are you pulling with (driving) and where are you going to want to take it?
I'm going to be building a trailer soon. It's not as sturdy as I would have liked but I couldn't buy the material to build one for the price I paid for it so it's what I will work with.
In your situation though, if a heavy trailer isn't going to work for you, you could sell it and buy a lighter trailer at least even if not pocketing a few bucks if you are smart about it.
As for the how, I propose WHAT. You need a decent suspension with enough clearance to fit bigger tires to give the trailer the cleanance it needs to go where you want. There are dozens of ways to skin this cat! My trailer (much lighter) has light vehicle leaf springs which are much longer than trailer springs and offers more travel and are fairly soft so it shouldn't rattle itself apart. It can be as simple as a spring over conversion with original suspension to a torsion axle conversion with drop/lift spindles to a custom trailling arm setup. Or if you aren't really going off road and just rough roads you may not NEED to do any of that other than what you find visually appealing (ie. Matching wheels and tires to your rig)
As for what goes on top of the frame, the sky is the limit!! Build with aluminum, build with wood, go all out or K.I.S.S. there are endless possibilities and no right or wrong way. What functions do you need it to do? Want it to do? What are you good at? Do you have more time or money available?
No go forth into the depths of the internet and be inspired! I've made a bunch of sketches of what I plan to build and am pretty sure what I actually build will be different yet!
For what it's worth, Cappy at 410 Expeditions has some videos of his trailer build on both youtube and his website. Excellent starting point and my primary inspiration ;)