Now for some actual advice:
If you want to talk about this, PM me. I'm actually considering something similar in similar areas.
Get to know the areas before you purchase. You don't want to purchase in an area you don't feel comfortable visiting. Know that some construction types won't want to go into your areas to do repairs. I know some contractors won't visit certain zip codes.
As an example, I think Berkeley has streets that are mostly comparable to each other. It does seem to have 30K homes renting for $750/month. The way I see it with those is if you do the legwork yourself, you pay off the home pretty quickly. I would assume repair costs as a higher percentage than a home in Chesterfield, but it will certainly be a lower dollar cost.
You are probably aware that most people talk in terms of dividing lines, but those dividing lines are not strict. For example, people say don't go north of Delmar. While true in parts of the city, there are $500,000 homes north of Delmar, especially in the Western half of the city. Point being that generic dividing lines are not useful.
Interestingly, I looked at moving to one of these ares in north city for the beautiful architecture. Despite being more dangerous than where I live now (Maplewood), the rent was higher! and I don't mean like $5 higher, I mean $100s higher. You might be undervaluing the rent you can charge.
Also, this should be useful for you -
http://www.stltoday.com/st-louis-area-murder-map/html_3bf7a0a8-0440-5ada-aacf-11e4314a9956.htmlThere's also a crime index somewhere on stltoday. Notice there aren't a whole lot of murders between skinker, Delmar, 170, and 70. But those property values are still really low.
In general, I'd agree that robberies and thefs are the bigger problem. The likelihood of being murdered is incredibly low. On that map, about 99% of them are either gang or domestic.