Blocking out your map is definitely something you should start with. This will give you a much better idea of what props you'll need, and how much work it'll take to get it finished. You'll also be able to spot design flaws allot faster, without having to redo whole sections of your environment. Mockup ftw!
From what i can see in your screenshots, all the buildings look very square and equal. I'd try to get that changed. A good example for this is
Wesley Tacks map Styx for off limits.
The starbucks windows definitely looks best from all the things you posted. It has some nice details in it, and i'm kinda missing that in the other pieces. Especially the wall is missing something imo. Some subtle dirt near the bottom might be enough to do the trick...
But for now i'd really focus on getting the layout in order, before starting to work on smaller props
cheers
J