Even if it was "properly" abandoned years ago, the possibility remains of soil contamination below the tank.

The first thing is to make sure the soil slopes away from the home and gutter/downspouts are in working order.Other than having a good working sump pump, other methods I know can be hit or miss in effectiveness.Some people have had good luck installing a french drain system in the yard to carry away excess water to the low point of the lot.

Are there parts of the city or zip codes that are (generally) more or less likely to have foundation problems due to the type of soil/clay/stone houses are built on? 

-Getting outside the real estate focus a bit here, but there are a ton of resources out there for figuring ranch profitability:https://extension.sdstate.edu/...https://texnat.tamu.edu/https://www.grainews.ca/livest...Obviously ranch profitability has several variables (rainfall, forage types and health, soil health, cattle prices, feed/hay prices, etc.), it seems to me one of the biggest factors that are controllable is the initial investment in land.

As I studied the NHD report, under the Geologic section, the expansive soils and subsidence sections are marked "in high" what is this mean?

The timber areas are often is mountainous areas where the soil depth is shallow and lots of bedrock underlayment, not exactly good areas to perk. 

The soil in the yard tested at super fund clean up levels.

@Sandy Uhlmann , I just uploaded the Standard Purchase & Sales Agreement for you and anyone else who needs it.

The area is sandy and there is no actual hole, but I'm so scared its the beginnings of a sinkhole. 

The soil got cleaned and the whole is filled, but the bill is not paid yet.