You are looking at potentially fair amount of work. If your rivets are working their way through the belly skin, then you could just add rivets and call it good, but to fix it correctly, you should drop the belly pan and replace it with new sheet aluminum. This is not rocket science on a 70's era trailer, but you will have to remove the lower belt line, then the banana wraps, and then the pan. Once that is done, it is just a matter of using the old corroded sheets as a pattern, cutting some new ones, and riveting them back on.
Chances are, though, that once you drop the pan and take a look at the outriggers and cross members, you will have a bigger job to do. If you are lucky, it will be only the wheel well outriggers that need to be repaired, and you could just twin the rusted through outriggers and seal them up. If you are unlucky, then every flange that makes contact with the bellypan will be rusting away, and the rear cross member at least will be disintegrating, and the floor will be rotting all around the perimeter. If this trailer has the original axles, then you might as well plan to replace them while you have the bellypan off. To completely replace an outrigger, you need to get access to the top side of the bolt that runs through the outrigger, the floor, and the C-channel of the shell. That requires removing the lower most inside skins (which requires a lot of disassembly of the interior.
You should stand on the back bumper and bounce up and down, watching for relative movement between the shell and the frame. 70's trailers are notorious for rear-end separation, and it would surprise me if your outriggers are rusted through, but there is no other rot or serious rust to your frame. If you find that this trailer has rear end separation, and you aren't prepared to do a shell-off restoration, then I would say that you should keep looking.