As far as how much sanding you need to do and where to strip again depends on what you are working on and what you want to achieve. Of course the rusted areas would need to be strip, repaired (with metal) ground down with a grinder (about 36 grit), epoxy primed, filled and primed. How much your going to sand depends on what your substrate is like...if it's old paint, or lacquer, or weathered base coat.numerous existing paint jobs...etc. It should be striped. If your dealing with a solid substrate like a base clear that is still in relatively good condition or if the car has been finished in the past with a quality single stage paint and is in relatively good condition, you could sand the remaining of the body with 180 to 280 grit paper (I would recommend this over just scuffing the car, you get more bit with the epoxy if it is sanded properly)and epoxy prime.
These are still questions that would help you get and us give better advice.