1. I prefer to media blast the metal. Sanding would require several steps using increasingly finer grits, and proper technique so as not to mess up the surface.

2. Buffing wheel dressed with a buffing compound. I've buffed aluminum ribbon covers to the point that they have a mirror finish.

3. What method will you use? A rattle can or an air gun?

Although Uwe is most likely correct when it comes to best practice, good results can till be achieved by hand - though not quickly.  Best I can offer is that you treat the task as a labour of love and invest many hours of careful work.  Begin by researching how to strip the machine down so that all of the parts you wish to paint are separate from anything else - you can hardly expect to get a good finish with the parts still assembled to others.  Work slowly and carefully in removing the remnants of all existing finishes - unwise to paint over them since you cannot be sure that the new paint will take well over the old.
Touch-up paint for cars (even metallic) in spray cans can make a good finish (after the correct priming spray to the metal) if you have good spray technique - research and practise.  Buffing bare metal can be done with an ordinary electric drill fitted with a buffing wheel to which is applied a little buffing compound.  Compounds come in different grades, but I should first try the brown (Tripoli) general purpose one - a small bar of it should cost no more than two or three dollars.  Good luck, and let us know how it goes.

Wow!!  I didn't know compounds were that cheap.  And it shouldn't cost too much for a wheel to put on the end of my drill.  When I get those together, I think I might try my hand at buffing a few aluminum parts to a shine and see how I come out.  If I can do it, I just might try a whole typewriter base (several are made of aluminum), and make it look like something that goes with a Kirby Vacuum Cleaner.