The rust is not so bad, however, the mechanic is bad. You will never be able to believe what he tells you. The rust can be slowed down dramatically by spraying Fluid Film on it. Fluid Film comes in an aerosol can and is often used to keep rust arrested on wire rope because it goes on thick but crawls into every crack and crevice. Crawl under with a can and spray all the rust you can see. It even has a straw that allows you to aim at stuff you can see but can't reach. Hit the seam on the gas tank. I believe it is the best rust stopping product I have ever used. I have no connection to the product, but my Mercedes is wearing some.
If you see rust above the side of the rear bumper then there will be no metal behind the bumper...that rust is awful.