I have done body work, and restoration professionally. Take my advice. DO NOT buy that car, you will regret it. I guarantee, that car has already been painted in the past, to hide rust. The only way to fix the rocker panel, and lower rear quarter is to cut the rust out, and section in new metal, but even then, it's probably spread further inside the rocker panel, and body
I'm not trying to discourage you, but the only way to get all the rust out is to totally strip the car down, and either dip, or blast the body. You will never get it all out otherwise. Keep looking man, just trying to help you