Well it is kind of hard to answer your questions because we don't know what your skill level is etc.. .
I would grind all the paint off in the rusted out areas and see how bad it is , cut out the thin rusted metal and treat the rusted areas with a phosphoric solution to kill the rust. Then weld in patches and prime. I have made patches and pop riveted them in and covered with bondo when I was younger and on a near zero budget but the right way is to weld them in. I would sand the body down , bondo, filler prime and sand the rough areas. If you have a local cheap painter [like an Earl Scheib in our area ]let them paint it for you .It all depends on how small of a budget and what skills and equipment you have.I have also just fixed the rough areas on a truck then had the local paint store mix me up the closest match to my paint in a spray can so I could just paint those areas until my budget allowed me to paint the whole thing. Anyway hope this helps some!
