I would go with:
Wall with window
Wall with door frame
Wall Corner (L-cutout)
Wall Section (blank wall section)
Seam Cover (Wall-Floor / Wall-Ceiling L-bracket things, I forget what they’re called)
Isn’t fill-rate handled by Occlusion though? I think it is…
As far as the “repetitive” nature of modular designs, you can use world-space UV’s (this would allow you to scale your meshes w/o texture scaling issues) in your materials, and you can also use props. If you look at most AAA games, look, really look, at how they build the level. You’ll see that most everything repeats, but you don’t notice it due to either a) Props or b) Decals
The stairs of course would be separate objects, as I do not believe that there is a “Mirror” mesh modifier that could be used.
edit: Make sure everything is scaled 1:1 as it is in the real world 