How to clean large area of marble
Bar keepers friend is a decent idea.
Do not pressure wash!
Marble is soft and you will ruin it. My advice it to call a carpet/floor cleaning company, or better yet a marble/granite installer.  Even if they do not clean themselves they will tell you what to do or where to go.
Marble is soft and will soak in certain liquids badly.  Once you get it cleaned, which should probably be done professionally, have it sealed.  It isn't 100%, but it will help a lot.
If you can afford it, get it professionally done by a trusted organization.  Be careful of handymen and others who do not specialize in it.  In the long run a professional cleaner will save you a ton of money and frustration.
If you want really cheap and easy, recover it and wait until you can afford the professional.