Providing you are speaking about re-creating this image, here is an idea that is relevant to your use of Glare - relevant part is:
Baking reflections and specular highlights is a bad idea if you want to move the camera, the object or any of the lights. The position and content of the reflections and highlights depend on their relative positions
You should bake the base colours, apply it as a colour map and let Blender calculate the specular highlights dynamically. It shouldn't slow things down too much.
One more piece of reference is this post from the Blender Stack Exchange that asks about baking glass:
Baking is generally used to simulate effects like shadows, lighting, small details like bumps or roughness through displacement maps, bump
maps, normal maps, etc., the kind of thing that is either too complex
or too expensive, or somehow unwanted or not needed to calculate in
Baking means it will be pre-calculated and become static, so it wont
react when you change the viewpoint. So if you bake things like
transparency or refraction or reflections they will become static, and
seem to be "printed" or "painted" onto the surface of the object and
that is generally not desired.
See here What does "Baking" mean?
If you are using this image as a texture, like a diffuse image on a plane for instance and you want the lighting to dynamically light your scene while the glass would be a glossy color, I would suggest it probably won't matter significantly to have an incredibly high level of detail between objects, you aren't getting that close to buildings. If you are recreating it, then try to avoid baking the gloss if you are doing a moving camera and let Blender do it automatically. If you are just creating a still frame with this image as reference, go for it.
If you are using this direct image as like a background to reflect on something, you would want to enable the Import Images as Planes add-on that comes with blender, choose this file and then you'd have the image already set perfectly on a plane mesh that you can manipulate and use for whatever you need.
As for how to draw different textures to different pieces of the building, either you would create the windows as separate meshes, and apply the glass texture or something I just found this post about using Vertex Paint to draw different textures. Either Vertex or Texture Paint would allow you the control of the emission or glossiness you are looking for, and it will mix all nice with certain settings to make your image more realistic reflectively.
Good luck! If you can clarify what your project is a little more, I'll try and help more!