That doesn't just look like "some rust". I hate to say it, but I would consider that a lost cause. That rust has destroyed your frame rails. And that level of rust at the frame rails would have me seriously concerned about the rest of the car.
What are the floors like behind/under the seats/carpet? How are the rockers? Dog legs?
Anything can be fixed, but it will be costly to do it right, especially if you're paying someone else to do it.
Here's Pete Paraska's site where he replaced his frame rails and also found rust issues elsewhere on his car: http://alteredz.com/structuralmods.htm
If you're going to take on this project or pay to have someone do it, do it like Pete did. And be prepared to find more than you're expecting that needs repair.
Otherwise, look for another body in better condition and be very attentive to areas like frame rails, rockers, quarter panels, and floors.