I'd actually be more comfortable writing about a fictional city. But a big, dirty, dangerous city that's not on the map? *shrug* It might work, but that still pops up the question, 'What state?' does that question actually need to be answered or not?
The genre of this book is...thriller, I think. There's also a problem with your idea to get someone who's familiar with one of the big cities: I don't get out much and neither do the friends I do have. Any other big cities besides Dallas/Fort Worth were visited or lived in during their childhood.
I really would rather a fictional city. I would need a name for it though, and that I'm not so good at.
Hmm...I'm going to have a little chat with myself in the bathroom...