First off, there are two great organizations that can be of assistance to you in your efforts to rehab in the city.
1. Saint Louis Rehabbers Club
2. ReVitalize St. Louis
St. Louis is prime for rehabbing. Right now I think you can get the best deals in Fox Park, Benton Park West, Tower Grove East and South, and Old North. You can always find places in other South city neighborhoods, but there is still some housing stock in these areas to be rehabbed for a good price.
Also, you might not know this but Missouri has one of the most generous historic tax credits in the country. If you rehab a house to historic standards and the house is either historic or is a contributing resource in a historic district, you may receive 25% of your eligible renovation costs back in the form of income tax credits. This goes a long way toward making a project viable. And if you the house will be income-producing (i.e. a rental house), then you can also apply for an additional tax credit through the federal program.
Best of luck with your endeavor.