http://wiki.polycount.com/CategoryEnvironment in that you will find quite a few links and topics concerning what you want. of special interest might be the one from stefan morrell on modularity. to paint blending between different texture variations vertex is probably the most common one, but it isn't necessarily cheaper…
Yes it will. I would suggest building it with a texture for a single floor, then clone the floor and add the floors. I guess this is what you meant by tiling geometry :) . For variation/breakup on the texture try doing what chrisradsby suggested and use alpha decals. Fyi if you end up tiling the geometry, Unity doesn't do…