I think I'd go with channel iron, I-Beam, or tube steel.
Also note there is a structural grade of channel iron and a not so structural grade. With the structural channel iron, the ends are thicker than the webbing, and tapered. The other channel would be more like bent flat steel.
My little 8' Haulin trailer has the bent channel, and it makes a nice light utility trailer, but You'd probably want something more substantial for your car/tractor trailer.
Your Kubota is pretty small without bucket, backhoe, weights, fluid filled tires, etc. If the company is building the trailer from scratch, you might try to get the fully accessorized tractor weight and give it to the manufacturer. Also, whether you will in fact carry cars, and what type.