I have decided to use GUMSHOE as my rule set when designing this city. I do not expect this to be a rules heavy exercise; it should be very easy to move this city over to another rule set. I do, however, expect to use some of the assumptions in the GUMSHOE rules when writing the fiction for the world. As GUMSHOE is an open rule set, I expect I will make changes and additions to it to suit my city. I don't want to give too much away, but I am going to need rules for power armor. You can find the GUMSHOE SRD here, so it will be free to follow along when I do use rules.
I think GUMSHOE is going to be a good fit. After spending some more time looking at the rules, I am confident it can handle the level of combat I expect for this setting. It will have to be hacked a bit for a few elements though.