I'm assuming it will be solid paint and not clear coated so remember you will actually be removing paint not clear coat.
basically as JD says
when I've played with cars like this they're generally fairly oxidised and dull, swirled and deep scratches all over and without doubt areas of poor or very thin paint, such as rust spots, flaking cracked areas. for this, and the fact that I don't enjoy doing it for other people, stick to a a very, very mild abrasive product, something along the lines of SRP, lime prime, rejuvenate if you want to use a DA. This will do a great job of cleaning up the paint, but you've got to assess the paint is suitable before going at it by machine.
If you're unsure, stick to hand application with a glaze or AG HD cleanse would be ideal.
looking through your previous threads am I right to assume you have fairly basic experience with this? if so doing it for other people is very risky. You need to get the expectations of the owner and make them aware of what is possible, and what you are able to achieve. Sometimes these are fields apart