It depends on the area you live which sealers are available. Call a local marble company to see if the sell impregnating sealer. You can also use an enhancer to seal tumbled marble, this will give it the appearence it has when it's wet. Most ceramic tile distributers should have a line of products for marble also. Two coats of sealer would definately be better than just one.
Denatured alcohol is usually good for clean up purposes.