If I had access to that trailer for that cheap I would jump at the deal. I weld (and collect "scrap" steel as a result) and would do all of the work myself. If you have access to someone that could weld I would say that is almost as good.
In regards to the side rails, I am of the understanding that their purpose is to prevent flexing of the angle iron frame. I would straighten those rails out provided that they were still structurally sound (not rusted away, etc).
Your truck may have a tranny cooler, but hauling a loaded trailer can still get that transmission hot. I would invest in a transmission temperature gauge.
I cannot recall what year your truck is, there should be a plug just in front of your transmission oil pan. You can remove the plug and use the hole to gain entry for the sensor. Then you just need to ground the gauge and find a KOEO live wire (behind the dash). There was a spot on my dash that I drilled out and mounted my gauge there.
I was surprised with how quickly my transmission fluid temps could get to the warning stage under the right conditions. There are some temperature charts online that will tell you the different temps that affect the transmission oil. Those charts will also tell you how fast your tranny will fail if you surpass those temps.
Ditto on braking those axles. Under normal conditions you can stop that truck and loaded trailer okay, but if you ever need to stop quickly then you are getting into a different territory.
As was stated before, get some new lights when you are rewiring it. There may be a minimum amount of lights that you need (brake, signal, clearance, etc) although there may not be a maximum. The basic rule of thumb is that whatever lights you do have all have to be in working order.
Do all of your light-work at once and then you will know for sure that your lights are good to go. Do not solder or shrink wrap any wires until you have checked the lights and their function (brake, signals). That way you won't have any work to undo if a light is not functioning properly or at all.
If you can get access to some 1/8" plate steel, think about building a lockable box just in front of your trailer bed. It will come in handy for storing your chains and load binders, and you may even have enough room left over to store your saw, gas, oil, etc.
If you plan on getting a winch in the future, build your box to accommodate the winch and battery, unless you plan on building a harness that you can run to your truck when you need it. When you are not using the wiring you can store it in your trailer's box.
Also, if you plan to haul wood that is higher than those (14"?) sides you can build some stake pockets and weld them onto your trailer. If you choose to do this, I would opt for steel stakes (instead of 2x4s) as they are less likely to break.
For moving your mower, that tractor, or anything else, figure out how many tie-down points you need and you can weld those on as well. My equipment hauler/wood hauler has some monster ones at the corners and 4 (2 on each side) part way from the front and back. The side tie-downs are for chaining down my wood so that I can run a chain across the center of 8' logs. It is a 16' trailer, but sometimes I cut the logs into 8' lengths.
For loading that tractor, it may be helpful to have some simple (and inexpensive) trailer jacks at the 2 rear corners of the trailer. You can buy the type that are pinned into a short 4" pipe. The pipes come with the jacks. When not in use the jacks can be left in that trailer box you decided to build.
Your trucks hauling limits are determined by a few different factors, one of which might be the fact that your truck may not have originally been equipped with a hitch system. I for one would not want to be pulling any serious weight off of a bumper mounted receiver. Hitches come in different classed from Class II or III up to Class V, with Class V being the most heavy duty.
Hitch system ratings can be found online as well. Find you one of them online charts, compare the different ratings with the heaviest load that you may be pulling, and ensure that all components of your hitch system are suitably rated.
My system is a Class V (I drive a diesel) and I have a set-up for a 2" ball (Class III as I only pull my boat trailer or my smaller utility trailer with it), and another Class V heavy bugger that holds a 2 5/16" ball. The only other ball size that I come across around here is 1 7/8", but I will just swap it in to the 2" ball set up if I ever need it. I put 2" couplers on all of my trailers, even the ones that I pull behind my quad.
This post is getting to be some past long so I will shut up for now.
