One thing about the grain, you need to change some settings in the atmosphere editor. in the light tab, push the ambient light slider towards sunlight 60% at least, and right under it, push the ambient light from... towards sky, 60% as well. Should get rid of the unwanted grain.  Also use more AA, the render you show has soft shadows that beg for higher anti aliasing settings, but I guess it's only a preview render.

Now about your textures, you say they are procedural, and not bitmaps, right? Try different space coordinates for them, try object standard, parametric, try reducing the mat size, or doubling it. the worn red wall to me looks too bumpy. Also, you could mix it with a dull white & grey material that would simulate dirt and age, and drive the mixing proportions by a fractal function.