I agree with Eric, most of Philly is not at risk of major flooding. Aside from 1 instance in the last few years when the Schuylkill river flooded homes along the river banks there isn't much risk of major flooding.
Philly is very unique, especially with the new construction tax abatements of the last decade which caused a major push for new developments and some areas faired better than others.
I had a new investor buy a very nice 4 bedroom in Brewerytown which he rented out for $2,550/month. The first tenants were in their 20's and I remember the father came to help them move in on key exchange day and he flipped out because of what he saw on the 2 blocks he walked past to get to their street. He wasn't from the area and didn't realize that starting around their street the property values changed dramatically and about 2 blocks in the other direction you had some of the nicest eateries and brew pubs in the area.
So yes, Philly is a unique city! Good luck and shoot me a message if you need anything!