1) If you apply High-Build wet on wet on the etch (or within the manufactuers open coat time), you don't have to scuff. That said, Etch dries rather quickly, so who knows. You probably going to be told to use Epoxy primer instead.
2)No, you do not need to prime the rest of the car. Unless you burn through to substancial bare metal, a good scuff with 400-600 depending on the paint is fine. To be safe however, Id use a sealer. It has minor filling abilities and provided holdout.
That said, make sure there are no problems in paint thats already on the car. No use painting over cracked, or peeling surfaces