Welcome to the forums!
Go to the "Portal" tab just under the Air Forums logo. From the Portal, you can find and download the buyer's inspection checklist. Granted, with a gutted trailer, there will be a lot less to inspect, but it will give you a starting point. Also on the portal, there is an inspector locator tool that may help you find someone in your area who can come and look over your trailer.
There are many threads here on the forums that describe members' rennovations from start to finish. There is also a thread that lists many of these "full monty" restorations at the following:
Another resource is the Vintage Airstream Podcast (the VAP). The early episodes discussed a lot of the finding, evaluating, repairing, restoring type of topics. You can buy all the old episodes and listen to them as you commute, etc., to passively absorb Vintage Airstream info.
"Shop" manuals are available for trailers from the 1970's and newer. They aren't exactly a step-by-step rennovation guide, but they do go into a lot of detail about how to fix pretty much everything that needs to be fixed, short of a complete shell-off. Most importantly, these manuals have the wiring diagrams for your trailer. As mentioned in one of the other posts, if you have a wiring diagram and a volt-ohm meter, you can track down and troubleshoot your electrical system.
As far as where to start, physically, with your rennovation, the first decision you have to make is whether you need to do a shell-off. This means figuring out whether the new floor you see was correctly installed/repaired, or is a completely superficial bandaid over a rotten mess. If you find that you have a lot of rotten floor, the decision is whether it can be patched, or needs to be completely replaced. For complete replacement, I recommend lifting the shell--gives you a lot of options on the repair/painting of the frame, installation of tanks, replacement of axles, etc. Search for "gantry", and you will probably find some threads discussing this.
The trailer is much like a house. The first thing you should do is fix the leaks (think vents, windows, doors, plumbing vents), so that water doesn't continue to pour in, and then work from the foundation up, ie., subfloor/frame/bellypan before any attention to the interior. Axles will need to be replaced, but they aren't the first thing to do, in fact they could be done last.
Just manage your expections: buying a 40 year old trailer is much like buying a 40 year old car that has been left parked in a field somewhere. You are going to have to do a lot of work on it to make it safe/functional, let alone "nice."