I am working on my 2nd trailer. the first was a basic one The box kite it was build on a free boat trailer. It need some welding but I did non and they broke and I had them welded later. Now the tire rod is bent and the cost was equal to a new HF trailer frame.
As you work on those houses you will have spare wood or know where you can find some. you can start with a very very simple build walls roof no galley. latter you can add a galley, later you can improve on the doors, later you can. And once you are ready and under stand what you want out of the trailer you can flip it's sale to someone who want to work on an existing one to learn before they do a new build.
I put on new bearings, wheels, tires, mine was on a boat trailer. I used found material as wall as new. I found a aluminum pallet, I bought kiln dried 2x4 from HD. Plywood for the floor any way it took me 5 years to build it with many things getting in the way. When I was close I found out about a gathering that would be just over the mountain and finished my build the night after they started. I have pulled it to New Mexico and back and up to northern CA many many times. Stayed at 9 KOA's through out CA and AZ. I got new doors several years ago and now I am farther into this new trailer faster, because I have looked at hundreds of TD's and old trailers until I kind of under stood the basics.
I have new concepts a curved front left to right like a utility trailer, Saddle bags behind the wheels instead of wheel wells, Doug H. showed me how to make a Trolley top as did many others. I am saying this only because you are in the perfect environment for building. There are ideas every every where, there are people from all walks of life, from Cabinet makers to draftsmen, many of which have been there and done that.
You want to start ..... Start asking ....... If you can show pictures of where you are at that is worth you know how many words. You can start with an old trailer is you wish but new will cost you just as much over time. If you know how to fix up houses you have built a TD for each room in that house. When people went to the national gathering they came back and were ripping things out of their trailer because they found a better idea there. Nothing says you have to keep your trailer the same way you can change most of it when ever you like. As long as you have time and already have the parts ready to put on.
Talk to your buddies, find out what TD or TTT folks are in your area. The first gathering I went to was in a tent. We had the only Prius there, but now we pull with a Carolla.
Best of luck. If you a looking for idea look on my gallery and many others.
Ron Dickey
Los Osos, CA