Painting a machine, if it's well done can save its life. Machines with huge gouges (i.e. aluminum showing, etc), or minor aluminum rot can be rehabilitated. To buy one that's cosmetically in decent shape to repaint, I wouldn't do it.
It's a big job. I have one in pieces at the moment. It's been in pieces for a year and a half. DH just finished stripping the shellac off it. Dave McCallum suggests using the black base paint as a base to paint on, so we may not strip further. The body shop at the dealership he works at now has said they'd paint it, and he wants to go that route so I'll have a "perfect" one. They've painted a few sewing machines before, and he knows how much "orange peel" makes me crazy. I guarantee I'll spray orange peel, I did it to a couple of motorcycle panels and had to sand it all out before I could clear it, then again to get it out of the clear, etc.
One tip: Ask whoever's doing the painting, if it's not going to be you, if they want to prep it, or if they want you to prep it. If I was painting your machine, I would want to do the prep so that I knew what was done. If you sand too much or too little in one spot, vs another, it will show up in the final product. A professional will have the tools to do it more quickly than you can, and while you'll pay more for them to do the prep, there's a good chance that unless you have done this prep before, professionally, that they will turn out a better result.
The paint and materials cost under $100. You're paying for the skills and the tools of the trade. Just like we would like people to pay for our skills, so should a bodyman (or woman) get more than a living wage for their work.