It's very hard to say without seeing the car in person. It's hard to think that if the frame rails are as bad as you say, that all the other rust spots don't have gaping holes in them also. Are the frame rails just rusty looking or are they crumbing away? Surface rust is not a problem, but if you can poke a screw driver through the rail you've got trouble. A rotted out car will be a pain in the butt! Do a good inspection and let us know what you find. Good luck.