I ask because having lived in SoCal my whole life, I havent really been subjected to any seriously rusted cars. I may be looking at this 65 880 Custom this weekend. Body looks decent as do the floors (carpet is out) but it looks like it may have sat with the rear window down for some time. The rear floor is gone, literally.
This is impossible to answer in the abstract. Show us photos of the rust and we'll tell you. If you posted photos, they're not showing up.
I would avoid this car. You haven't said what the asking price is, but no matter what it is, you can likely find a similar car in better condition for the same or similar money. So much of the car is gone here that I would question the safety of driving it. How much other rust is there that we can't see? There is lots of rust behind here that you can't see, and it's probably like this all over the underside of the car. I wouldn't even bother to go look at it.
Funny, I saw the original pictures. Looks like a New England car. I guess the answer is on your fabrication and welding skills. It will take many hours to fix it right. It will be real expensive if you can"t do it yourself. I always try to get the best body I can afford. That being said I fixed worse
That soccerball looked like it was levitating. +3 on the "what is not visible." If those are that bad, there's no telling what else is bad without a proper inspection.
I once had an old car restorationist tell me that rust is like a glacier. You see the 10% that’s at the surface. It’s the 90% you don’t see that costs you real money. My prospective on rust has changed a lot since I moved to the southwest from Chicago. Jaunty and I are spoiled. I would say try to find the best example of what you want, and pay more for a rust free example. To Alfatar’s point Pre EBay and Craigslist I bought my share of rusty cars cause that was the only thing in my area. I really examined those cars to find the ones with minimal rust. Not only do you have body rust to contend with, think about what’s underneath trying to turn bolts, and remove parts that sometimes you just have to burn off. Rust is a pretty problematic issue.
If it was only the wheel arch area, I would say that's acceptable. Needing an entire cargo area floor is too much. That would cause me to wonder how sound are the frame rails and other parts...?
Floor boards like this aren't a big deal, but there are better examples to be had. I'd rather spend time finding those examples than fix the rust.
I'd say "too much rust" is relative to region. For me, the photos you have shown are way too much rust. I base my opinion on years of living in a very dry and arid part of the country. On the contrary there are folks that live in other areas of the country, or continent for that mater, that would look at this particular car and say, "I've seen people restore much worse" It all depends on region and as ALFATAR mentioned, it also depends on your skill level when it comes to metal fab work.