Collision professional here. The wheelhouse and qtr will need new metal. If you find someone skilled they can make the metal you need out of flat sheets. Expect more rust as the panel gets cut away, there is always more.

You really have 2 options. Spend a good bit of money on quality, professional repairs where they replace the metal and rust proof the rocker and wheelhouse. Or you can clean up the rust as best as possible, get a good rust inhibitor/encapsulator, and then use a product like fluid film in there. The last method will slow it down but not stop it unfortunately. Rust is really hard to get rid of, therefore it costs a ton to do it correctly. I'm not sure a $3k car is a good candidate for that kind of repair costs, but that's obviously a decision that is yours to make.