Just the outside to get the rust off. It shows through the Magnum 500’s and looks crummy.
Any problems? Maybe some high heat black paint after
No problem with doing that, or some cast gray paint
No real problem if you protect the machined inner surfaces from the blast. Soak them in a product such as Evapo rust is another though more expensive option. Depending on how you plan to use the car you can treat the bare steel after returning the visible exterior machined surfaces, paint it a high heat cast blast (grey rattle can paint) or just paint them black.
Originally may of the Ford and Mercury products that were ordered with wheels that exposed the face of the drums or calipers were painted (the outward surface) with a big sloppy brush and some black paint
A better approach is to use a plastic oil drain pan with Evaporust. A 6 quart drain pan is perfect for one brake drum and a gallon of Evaporust. Brake drums come out looking like new after 24 hours. Less mess than blasting and better results.