Although the "fix or repair" sounds good, sounds easy, it is
more involved, but not impossible to do.
Sure you can throw some boards across a open or rusted span...but is the structure underneath sound and safe to stand on, to carry the weight of anything you place in that area?
That is the question needs asking and finding a answer to.
If the tack area is this rusted you can bet I would also be under that trailer looking at every cross-member, support beam and frame component to make sure they can and are worthy of having your horse stand on them in trust his transport isn't going to maim or kill him going down the road...
Many horses fall through floors that "look" good.
You bought a old trailer...
For $1,500 I would expect it to need some work, some serious work
, to be road safe for any horse and that is fine.
Just do the checking carefully and fix it correctly so you and your horse are safe when out and about.
Expect to invest time and $$...even a trailer that is just a couple of years old needs maintenance done to it that is time and $ spent...but the rewards and enjoyment that $ can give are so worth it.
Enjoy your project....and do share some of those pictures.
We love to see projects evolve and be completed for our "friends" to enjoy!!