Cycles. Texture bake problems.

Hello, blender community.
I made a simple model of building (with no inside and small details). Then I painted it by textures and set up materials. I’ve optimized my model and now I think, I should to bake my textures in one (color, specular, normal maps), but I have a couple of problems here.

  1. It’s difficult to unwrap whole model (big building) even on 8192*8192 texture.
  2. When I bake my textures in one, blender gives an error in console: “ImageManager::add_image: float image limit reached 5, skipping …” (I have about 50 materials and at least 1 texture in most of them.)
    So, how do you deal with this problems? Should I keep separate materials?
    How do professionals finish their models? For example, if they want to export their model into UE4 or Unity.

Model renders:

Archive with the .blend file: