I would give some thought to the trap design. I do not know what your heating season is, but that trap will likely go some time without receiving any water. In order to avoid losing your trap seal, you will need a self priming trap. Though, I have seen a new device that uses a sort of curled up tube that allows water to pass, but then curls back up to seal the trap. I forget what it is called, but perhaps someone else will know. I am not sure about this product's code acceptance status, but it seems useful for certain situations. As far as the deck, If you are going to take the time to do a mud deck, you might as well install a proper shower pan. That way you can just run a hose bib in the room right onto the floor.