Good looking trailer, I like it.
Fenders not only protect others from flying debris, but it somewhat keeps your tires out of the sun. The sun will dry rot or crack them fast if not kept under a shelter or garage.
Go ahead and put your sides on and then build fenders that mount onto the uprights you already have and attach to your sides too. You should get good support this way. No need to add more uprights for fenders.
If you have a shop close by that can bend plate, measure the trailer to figure out your width and length you want, have him bend you a 1 1/2 inch lip to bolt the fenders into the sides, come out as wide as you want, then bend a drop lip an inch or so to give the metal strength/durability. Have him bend 1 piece as long as the entire fender including front and rear drops. When you get it home, cut out a v on both the inside and outside drops so you can heat across the fender and bend the end down to have it bent into a 45% angle, or whatever angle you want or think will look good. Also mount your tag sideways on the back drop on the road side of the trailer, visible from the rear by any cop.
Build them heavy, they will become work benches, sitting benches, standing steps, and a blown tire will murder them if built light.