How should I fix a great amount of clipping, overlapping, and z fighting?

I have a project in which I am to create a model of my high school, inside and out, classrooms included. I expect it to take me quite a while, but for the time being, I am trying to create an emptied out, boxy outline of the shape of the building. However, do to many parts of it, both now and in the future, I need to fix the issues (a result of my carelessness and little actual skill) described in the title. What might be the best way to do this? There are many faces that I cannot even select.

The blend:

Okay… Sounds like a laborious process, but I’ll try again tomorrow.

It’s not that bad. You can do quick extrusions from selected by ctrl+left clicking. If you have a vertex selected, it extrudes an edge. If you have edge selected, you get a face. Could also do wall/window placements with loop cuts after you have whole faces to cut, so don’t have to subdivide edges only.

That is of course if you need thickness on the outer walls in the first place.

First try to work to scale. Your rooms currently are, depending on what you like, ~28 meters or ~92 ft high.
This uberdimensioning causes the the visual clipping int he viewport due to the camera frustum and the limited precision left if it has a big stepping.

As for your model, return to a previous version where you only have either the outer walls, or inner walls and use the solidify modifier to generate your walls.

But what you got now is a complete mess.