I dont know how to select one face, but I can tell you what I have done to get around this.
Make a brush hollow. This allows you to make a cube or other shape. Then select one of the walls and duplicate it. Now reduce youre grid size by one setting. Reduce the width of the original and duplicate wall by half. Place the duplicate wall directly next to the original wall. Voila, you now have an interior and an exterior, though it is a bit cumbersome. As for adding multiple textures to a single face, I would make brushes representing the area you want and use the csg subtract tool.