I group brushes or put them in folders and copy and paste everything that repeats. In architecture that is usually a lot. I try to use as few pieces as possible and use them over and over again. In Maya, I also found it useful to place the (left, front) corner edge of my meshes on 0,0,0 Position, as that will make them snap more easily than when the pivot is in the center (hold v to snap and drag in UnrealED). Later on, I replace the BSP/CSG with the actual models, after exporting the BSP to use it as a template. Some BSP, such as floors often remains in the final version of a map.
This map must be extremely hard to work with, if you have so many actors. You might want to figure out small and large modules first and make a list + sketches of everything, before starting the in-editor work. I often also make a list of materials I need and start grouping my brushes/models accordingly, so I know which assets belong in the same material group. I find organisation extremely important when working with the Editor, so I use a lot of Groups and Folders.