Marble will work on the shower floor. Apply a sealer to the marble. I usually use an epoxy grout, which doesn't need to be sealed and won't discolor, but a properly installed non sanded concrete grout will work fine (sanded grout may scratch the tile). If you use epoxy, grout smaller areas than you would with regular grout then clean up the excess. If you build the shower pan, I would recommend using a rubber shower pan liner (instead of felt paper) under your concrete (there are videos to get some info on that and you may need to change the drain). When you attach your backer board to the walls, keep your screws about 10" or so off the floor and I would also recommend using Schluter Kerdi membrane on the walls (with a non modified thinset).