I have done a few marble floors over the years, around 100,000 sq. ft.. If I were you I would not do this I love OP machines but I don't think you will get the results your client is looking for. This best way is with a high speed buffer 1,200 RPM using a neutral cleaning such as super shine all by Hillyard. Pads are graded by color white is the lest aggressive, red green and black is a stripper and should never be used on marble. Remember this, marble is a very unforgiving type of floor and if you make a mistake you will have a night mare trying to fix it. I would never just clean marble without finishing it because the finish is what makes the marble look great, often times just cleaning will leave it somewhat dull depending on the base finish. That being said 30 years ago I would have just jumped in and done it.
If you do decide to clean it, look for a stone cleaner used on marble. It will be an alkaline, not an acid cleaner. Acid dulls marble. Don't be like all too many hired housecleaners who try to clean it with vinegar. I would use a thick, rather than a thin cotton pad.