Be sure to include product liability insurance in your to do list.

I recently bought a new trailer and applied for a title using just the bill of sale and certificate of origin. That's in VA, but should be the same most places. Even new cars and trucks are not titled until the first retail owner does so.

Go around some of the trailer sales lots and see what info comes ON the trailer. VIN tag, maybe tire size and pressure info, EW and GW ratings, etc. You can either have tags made, or "burn" the VIN in with a welding rod.

Most of the hardware is direct from China. Should not be hard to find axles, etc.