We recently restored a cattle trailer, an old Sundowner. After we got all done we realized it might have been less expensive to have bought a newer one.
You might want to take it to a trailer repair place and get them to give you an assessment of what will need to be done to make it street legal and safe for your horses. You might be surprised.
Not only do you need it to be structurally sound the metal under your floor boards has to be safe. Then the axles and wheel bearings, wheels and tires may need to be replaced. It will need working lights and brakes. All of those things are much more important than the paint. I don't think you can use plywood to fix the floor either....you will probably need new floorboards. Getting the rust and old paint off before repainting required having it sand blasted....and that was expensive too. Then we needed new fenders...ours were too rusted through to fix.
I hope yours will be easier than ours was....although it looks in much worse condition in your photos.!