Some stuff that comes to mind.
Its been sitting there for a while look how much its sank.
You will likely need brake drums and backing plates.
Wiring is likely green or brittle or both. If its questionable replace it ALL!
What is the trailers weight since its "custom" add the 3 horses you want will the axle ratings still cross the scales?
I'd pull the hubs and repack/replace before going far its sat so long. Simpler to do when its not on the side of a 400 series. (drove wrecker enough to see lots of chewed spindles)
Personally i'd take your spare and borrow a buddies and bring it back on a single axle and worry about tires in your driveway.
Is the trailer a frame design or a box design? Windows may weaken it . . .
The spring gate lift can be added simply.
Pull that out of the dirt, CLEAN it up and it'd likely pull 1500 lol without any work. Good rubber, repaired, windows, paint and papers would pull $3000 easy I'd bet (here anyways)