I'm not worried about textures or lighting until everything is in place. Not that I'm guaranteeing a grimy, damaged mansion. And making changes in Photoshop is not the way to go about it. Textures are generally in 512x512 patterns set to .25 scale that repeat over the entire brush. Making a new texture would just cause the same damage to appear multiple times on a wall. There are 2 actual solutions: make the walls a displacement with a blended brush to paint damage onto the walls(done in the new inferno), or place decals of damage onto the wall. The former looks better, but will cause the walls to no longer seal the map.