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

And the septic tank, which had to be replaced was positioned in the yard that all of the drainage ran under the back house (which was set on concrete block piers) and was stagnant underneath, preventing any of the inspectors from inspecting underneath.

Thanks @Claude BoironMy intention is to build up a lease of all of the "extras" that I find from experienced landlords that a lawyer would not include, then go to a lawyer to pour the foundation underneath it.

Morris Investments runs a Holding Company LLC which manages multiple LLCs underneath (see one LLC per approx 150k of value).

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

Our mailers to facilities with managers often never make it to the owners because the manager takes one look at it and thinks, "I don't want my job to be sold out underneath me!"

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. 

There is a colossal effort that goes on underneath that tip!

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.