I love truck bed trailers. The reason I think people make them is because it is easy and cheap. They have a busted up truck and chop the front off, and fab a tounge. I agree that your way is going to be tougher, but I also think that you should go forward with it.
However instead of building a frame, I am sure it would be much cheaper and easier to buy a rear clip of a truck. It does not need to be a checy 2500 HD blah blah blah. I am sure any full size truck would do. Last time I was at the junk yard there was miles of old full size american trucks. You don't need the bed to bolt up exactly, you were going to be fabbing the entire thing from scratch, I am sure welding up a couple of mounts would be a piece of cake in comparision. The leaf spring, shock mount, axle and brakes would be the real time and money saver.
since you are not using any of the running gears, you can put smaller tires/wheel on it to keep the overall hieght lower as well. Since you are making mounts you can also mount the bed lower than stock. Most trucks have pretty tall stand offs to mount the bed.