John McGaw
At worst you would have to run a diamond hone on the floor to rough it up and even out any humps (or pay a floor company to do it). This will give enough 'tooth' for the thinset to stick. Porcelain tile would work as a surface but you'd have to be careful to purchase the sort which is rated for wet situations because, as somebody else pointed out, some tile is dangerously slippery when wet. The good stuff has a microscopic grit to it.

If I were doing it, I'd go for one of the plastic/rubber snap-together tile systems made for exactly this sort of use. It is relatively inexpensive and pretty much the only tool needed for installation is a dead-blow mallet. Even better, if you have some spares it is easy to replace pieces which might become damaged in the future.