We know of a master bodyman if you have ever heard of Dan Coleman or Danny Coleman his son they owned a shop and did beautiful work, we also know a master welder, and a great interior upholsterer... And the car belonged to our great grandmother...
But do you know a banker or a good loan officer? Paying somebody to do all that work is where the problem lies. If you can do it yourself, then go for it.
Very good suggestions here. Fixing rust is VERY expensive. I'm presently parting out a complete running 76 Stallion due to rust issues. The longer a car sits up the more rust you'll find in it. I'm not saying it can't be fixed because for the right amount of money any of them can be saved. you just may have to replace more of the body on some. As stated, there are always a lot of good cars popping up on this site, eBay, and Craigslist, among other places for sale. A lot of these cars are rust free or very little rust. You're way better off spending $2000-$4000 for a rust free car and it may be better optioned to save you even more money. When you get one of your body guys to look at it and give you an estimate I believe you'll find we are right and just looking out for your best interest. Good luck with your car, however you decide to go with it.
I might joke around a little about rust repair but i wish i would have done my homework and found one in better shape. I have better things to do with my cold winter days than to repair rusty torque boxes, floor pans and the dreaded leaking rusty cowl but on the other hand i'm saving one more car that might otherwise end up in a shredder! At least with mine i can drive it and work on it as well. If you need someone else to do the work its gonna cost big time!!!!!
Mine sat for 20 years and had a lot of similar rust issues that your car has. First off, if I didn't build my garage back in 05, it would still be sitting where it was rooting. I am a mechanic by trade so I looked at it as anything can be fixed. I bought a welder and learned how to use it (somewhat) and got to work. Now if I had to pay someone to do all that I have already completed, it certainly would be a different story. As has been mentioned rust is expensive to fix unless you can do it yourself. As an example - the things rusted on my car, a lot of it from just sitting that long, were: trunk floor and gas tank well (bought metal to repair) front floor boards (bought Mustang pans to make repairs) driver's side front torque box (bought a rust free Texas box) cowl vents (repaired with Mustang parts) quarter panels (bought 2 good used ones) rear of front fenders (bought used 2 rust free fenders) rear of one rocker and the front of the other (bought 2 rockers from Texas) hood lip (bought another hood) rain gutters (am removing the gutters and leading them in) Starting out with a more rust free car would have been my answer if I wasn't prepared to tackle it myself