With that much rust-through, I would never trust it to haul live cargo. You would need an experienced welder to repair it, and that person would also need to have a good working knowledge of load bearing support structures and stresses from traveling. Posted via Mobile Device
Floor is badly rotted in that one photo so it will have to come out and be replaced with more steel or wood. Once the floor is out you will really have to spend time going over that subframe. Each and every rotted piece will have to come out and be replaced. This, as mentioned above, will take someone that can weld to do. And I don't mean someone like me, I can weld two pieces of metal together and even make it look decent but I would never ever trust my welds in critical joints like on a trailer. There are lots out there like me so make sure you find someone better at welding, not just a friend that says they can do it.
For the doors you can just cut that out and replace with some fresh steel. Still takes some welding but not nearly as important structurally as the frame. It can be more or less scabbed in and covered over with some aluminum sheet to make it look good from the outside. That's a common fix around here on older trailers.
We do serious welding around here all the time, and *I* wouldn't have bought that trailer for $400. Mind you, I don't know what trailers are worth where you are, so it might be worth the investment in your area. You definitely need a welder / fabricator to fix that. You can save some $ by pulling the woood floor out yourself, but you will probably need a grinder to cut off the carriage bolts / screws in the floor.
A body shop could do the work also, but get some prices and see if it's worth it to you or not. You will need to specify exactly how far you want them to go -- ie, will you do the finish work or will they? Chances are the hangers / u-bolts are bad and/or what they are(n't) attached to.
Ouch. You are going to need A LOT of work done on that thing. I would take it to a manufacturing company that does sheet metal work. They are going to need to cut out sections, bend and weld in new sections. Especially the floor. It looks dangerous.
about 20 cents a lb at current scrap prices. Which actually may be a good deal if you can tow it to a scrap dealer without the rubber tires it should come in close to 2500 lbs X .20 = $500. You might turn yourself a quick $100 profit.
Before starting any repair work, the frame needs to be checked, not only visually but some good hard whacks with a hammer. Then check the bearings. Those can be an expensive repair if all four need to be redone. If you can get your money back by scrapping it that is likely your best best. To bring it up to roadworthy the cost will override the price of a good used one.
I'd seriously consider scrapping it out. That floor would scare me. I would also think the time & money it would require to get it road worthy & safe enough to put horses in would definitely be more than it's value or what you could pick up an already solid, safe one that may need some minor cosmetic work.