If you have a welder and a sand blaster, you can probably fix all the rust. I agree that the floor pans aren't a big deal. People replace them routinely. I used to use a wire wheel, but a sand blaster is quicker. Maybe use a putty knife and wire to get the big stuff off. First thing is to remove the rusted floors by drilling out the spot welds. I think then you can see the inside of the frame rails to judge. Try poking a screwdriver thru all structural metal. In California we don't have to deal with that. My 65 Newport was pristine when I bought it in 1991 in SoCal, with not even a spot of rust inside the rear bumpers. It didn't even get much from sitting outside in Atlanta for 6 years before returning to its home state.
Start shopping for a new windshield. They should be cheap and available since many rusted out cars around, but C bodies are rare. Remove the spark plugs, fill with oil, and try turning the engine over by hand from the crank bolt. If it is seized, it isn't worth much. If it works, it is worth several hundred alone since A-body guys use them for drag cars. However, I think yours has a different crank with a smaller hole for the torque converter.