You say the "house plumbing works wonderfully"; with the drains full of roots and soil, it's only a matter of time until they start clogging and creating sewage backup in the units, so I would say you have no choice but to fix this now, and to fix it properly.

A big factor has to do with what type of soil the slab is poured on.

Not sure about College Park but I know a 2/1 in Sandy Springs that is bringing in about $2700 per month from Airbnb and his purchase price was around $68k

The reason the slabs may be a concern is that the potential for the  soil to heave and crack from the clay in it. 

Most old houses they dug a hole in the ground on virgin soil and never put any footings in.

Old gas lines in this wall, steal 4” soil pipe through the house, load bearing support, moving electric etc… there may be a bit more to pay for than originally intended.

When she went to sell she didn't have a record of the oil tank being removed some 15 years earlier and had to pay a large bill to get the property inspected for contaminated soil. 

Directing downspouts further from the house or adding dirt to create slope away from the house to try to make the soil adjacent to the foundation less saturated with water and less likely to heave and push on the foundation.

Just wondering if you have any info on what's driving this market growth, how long it's expected to continue, how it's expected to survive any downturn in the general economy (there are many signs and predictions, but nothing really solid yet), etc.It's great to have growth if the soil underneath is deep and fertile.

You will need a soil specialist to come out and determine what kind of system.