I built alot of these in my day as motorcycle trailers to store camping gear and stuff. Might i suggest a few things.
1. make sure it is perfectly square so it tracks correctly.
2. reinforce the tongue, they are well known to collapse (fold up) under stress. I always changed mine out for tube steel.
3. i always liked the look of diamond plate covering the tongue area, its not that expensive and looks great, plus it helps strengthen that area.
4. alittle trick to keep it from chirping down the road as they all do. Take some small sections of water hose or some other rubber tube and insert the bolt for the leaf springs through it, it will help quiet it down. Some of the newer models came with bushings to do this but the older ones did not.
5. while you have the frame available in that condition use this stuff or something similar to treat the inside of the frame against rust.
Eastwood Internal Frame Coating Aerosol Black
6. Run the electrical (lights and power) and water now, its much easier.
Finally, remember to have fun with it, it can get frustrating at times trying to find material and deciding weight over convenience.
Best of luck...