We had african slate on the floor of our kitchen. Used a charcoal colored grout. Looked terrific. Previous owner did it and laid it on luan over an old linoleum floor. In the 7 years it was down I never had to re-seal it. So don't get yourself too worried about sealing. But I wouldn't go with a sandstone, that's just asking for it. The upside to slate with gray grout is you really couldn't tell just how wretchedly filthy it was at times! This is both good and bad!
The downside to the previous owner's crappy install was terrible lippage and a few tiles whose grout seams would creak a little now and then. Not enough to be cracking either the tile or grout though. We had about the same deflection. But it was a relatively small galley-style kitchen.
I strongly suggest you join the John Bridge tile forums and ask over there. Those folks REALLY know tile work.
I tore up my floor this weekend to the base boards. I laid 23/32 " plywood down. The floor seems really solid and it doesnt seem like it flexes at all. I put a glass of water in the middle of the floor and jumped up and down around it. It did even ripple. I have one question though. ive noticed some of the boards are thinner on the ends compaired to the middle. The floor was clean and theres nothing behind it. Ive noticed in one area thats theres about an 1/8" or less if a height difference. How do I correct this before I lay the backer board down.