While some of our warmer-weather members may be concerned with rust, in general, I may be a bit more familiar with it and can give you my take as someone in a similar geographic location. While body rust in general is concerning, the big issue is the frame and torque boxes (the channels under the body that the frame bolts to). Need pics of the frame itself and the places where the bolts tie the body to it.
These are the things I have learned in my battle with restoring a rusted Jeep.
1- If the torque boxes are rusty and swollen where they bolt to the frame, it is a big task to repair them. Not impossible, but it will require some welding skills and fabrication skills. Not a quick thing to fix.
2- The body rust on the rear corners and under the tailgate are also very difficult to repair. The corners are spot-welded on it the worse possible way, as is the threshold under the tailgate. To fix the rust in these areas requires you to do some pretty major surgery. Again, not impossible, but this is the second worse place to fix rust. On the drivers side, that rust repair will require a lot of work because it is both the corner and rear side of the tub.
3- Floors are pretty simple to fix with basic welding skills and the fender is an easy replacement.
4- The rust in front of the rear wheels (under the flare) is fairly complicated to fix. The flare will hide the visible damage, and I suspect that there is a pretty big area under there. To cut it out and replace is a bit of a task because there is a support channel on the inside that will also have to be dealt with. Again, not hard it you have some good welding skills.
So the real question is, are you willing to invest that much time and money to fix it or is the price low enough that you will just run it until it rusts completely (I give it 2 yrs tops in my neck of the woods - not too far from you) if, and only if the frame isn't compromised. At that point, it will essentially be scrap.