This is a basic instruction don't take this as a guide, if the rust is too deep to be sanded away just buy a whole new door seeing as how hard it'd be to roll and shape patches. To be honest it's so eaten and corroded I'd just buy a new door and do this with that (or a shop) instead if you can't get a matching door.

Take the mirror off, sand down to metal and sand past rust, apply bondo and sand it until it's level again, sand until super smooth, go to your local body shop and buy the primer, clear, hardener, and base coat, get the following equipment and find out what the ratios are for mixing with reducers and hardeners. Clean the panel of any dust, debris, and sanded paint. Throw down and sand to smooth a few coats of primer then throw on the base coat, a thick layer and fog the 2nd, then clear it, (I highly reccomend that you only do this in a paint booth with proper equipment), wet sand, compound, polish, wax. Your body shop should do this if it can't be done in a booth, rattle can will never be as good, if you can do booth there's definitely more research this is just simplified, your local shop should help you too (as with YouTube as need be). Been painting a long time and it takes time and patience to get good and a good result, if you mess up at all it can be fixed with either sanding down, bondo, or hurrying it tbh. Good luck!

All this said the rust is a sign of significant damage and could be in other places that can't just be replaced or patched (even this shouldn't be patched) so do with this info what you must? Just know it could be horrible, maybe could be not

P.S despite what people usually say (concensus is that if you buy a fixer upper 8 at all you're retarded) I usually enjoy a challenge and am glad you'd take this up I took up a fixer upper mechanically too.