Ironically in Texas and areas in the midwest.I do not see a lot of good deals in the Atlanta market at all.

I left the cast iron pipe exposed and just painted it the same color as the wall.

I am slightly concerned about the age of the house and what lies under all the sheet rock (possibly cast iron plumbing, old electrical, etc...) and don't want to be held responsible for someones child dying due to my neglect of DD and the house burning down.

The drain cleaner snakes or jets the line and rarely are they able to see the items flowing past.Also in some cases it's a combination, it could be an old cast iron pipe with a rough inside surface full of rust, which started to act like velcro to things that flows past, or a slight settlement causing a belly, which again impedes the flow, then the tenant pours down grease, pasta and bean, well, you get the idea.Most of the time you end up eating the cost especially if the plumber can't tell you anything.Now, you mentioned that the clog caused water to back up to a vacant unit next door. 

Currently here in Western Australia we are in the middle of a crash in iron ore prices.  

Vacancy rates will rise quickly.Maybe you could try an Iron A/C cage on the outside and a separate security deposit (maybe around $300 or so) to cover the A/C.The cages are ugly, but in many window A/C communities the residents understand that their neighbors (and their neighbors visitors) may be, shall we say sometimes "theft prone", and the bars can be explained as break in deterrents to keep their belongings safe.They also blend in better in a light color vs. black.If a prospect balks at the $300 and you want them, you could always offer to wrap it into the first 10 month of payments.

Ironic considering the name of this thread. 

As everything has to "season," like a good cast iron pot.

Maybe the cast iron plumbing is getting suspect on one, sell it if a better deal comes along.

I thought that was ironic.