I would take your trailer to a fabrication shop, a truck or tractor/farm repair facility, that is competent in welding. Especially for the frame, you will need a structural steel doubler repair welded into place. Depending on the area, this may be a flat plate or a piece bent into shape, mimicking the original contours.

The fab shop can also probably rebuild your doors and address the hinge problems. Welding, cutting, and shaping are what these companies do as a business. With their skills, materials and tooling, many could probably fabricate a whole new trailer. The proper tools, materials and knowledge, can effect the repairs in the fastest and most durable fashon. The cost of the expert repairs may be less expensive in the long run.

If you do have welding experience and equipment, they are also a good source of materials.

The materials available at most big box stores are not stout enough for frame or undercarriage structures. They are soft steel, of smaller dimensions and are best suited for non-structural repairs.

I don't know the area in which you live. A quick glance in our yellow pages under STEEL, detailers/structural, revealed several sources of steel products. Whether it be sheet or extruded products, a few phone calls may produce the leads you need.

Having a good welder in the family will drastically lower your reconstruction costs. Best of luck with your project