Nikola has sort of already answered your question about "suburbs one," but I think he didn't notice that it was in the sentence he explained ("out in what had been before the war the suburbs one could find buildings entirely empty"). The two words, "suburbs" and "one," are not a single phrase. They belong to separate parts of the sentence: "out in the suburbs" and "one could find." Here, the word "one" means "a person"/"someone"/"anyone."
Here's a way to clarify the sentence:
Out in the area that had been the suburbs (in the past, before the war), a person could (now) find buildings that were entirely empty.
Philip K. Dick writes in a rather unusual and creative English style, so you may find quite a few of his sentences hard to follow! The grammar of this sentence is deliberately confusing, probably because Dick is trying to imitate the way people think, sometimes very quickly and not very clearly. You'll also find that he sometimes makes up words, since he's describing a future where there are things that don't exist in the real world.
>> "for this day and age a one-half occupied conapt building rated high in the scheme of population density."
It doesn't mean that the place is full of people, but you have the right idea. It means that even though there are NOT many people, it is still considered "high-density" "for this day and age," because it is very rare for buildings to have many people in them. Even a half-occupied apartment is considered a "high level of population," because it is rare for apartments to have ANY people in them.