We're looking primarily at northern states with lots of rainfall and ground water.

Septic could be problematic since oils and cleaning chemicals could be discharged and then leach into soils/groundwater. 

Ground water seeping in probably would not meet that.

heating oil tanks are NOT regulated by EPA. you have to look at your state cleanup standards. 20,000 ppm is significant but the good news is that heating oil tends to be very viscous (unless you were using no. 2 oil) so it may not migrate very far. the cost dramatically increases if you have groundwater contamination and the groundwater is used for drinking water. you need to engage a environmental consultant, not a tank contractor.

.), two bathrooms, added a bathroom, fully redid the kitchen, refinished the hardwoods, paint, baseboard, trim, redid all of the plumbing in the house (above and below ground), water proofed the basement (internal/external french drains and sump pumps, and did all of the landscaping (mulch, new sod, removal of trees, etc.)

@Allan Smith I don't understand what people mean when they keep saying "water lines" but a sewer lateral is not just a water line, when it breaks it releases raw sewage into the ground water.

Usually, only heating oil USTs associated with commercial properties are regulated. thus, most home heating oil tanks may and often are taken out of service without any oversight from state regulators.Second- the rules that are in effect for heating oil tanks are usually less stringent than for diesel or gasoline tanks since heating oil tends to thicker and may not migrate as far. as a result, many states just require tanks to be cleaned out. they may also require the tanks to be filled with sand or concrete to prevent collapse. but sampling may not be required.Third- the key to determining if a tank has leaked is to collect soil samples from around and beneath the tank (groundwater also if groundwater is shallow). another quick way to see if a tank is leaking is to "dip" it with a stick that has a paste that turns color in the presence of water. generally, if water has gotten into a tank, it can mean oil has leaked out of the tank (although a certain amount of condensation may be present even where there is no leak so this is not a infallible test).Fourth- if there is currently a tank in the basement, ask the owner if there used to be a buried tank (a/k/a underground storage tank).Fifth, If the tank has impacted the soil, the cleanup generally varies from $25K to $50K though the costs will depend ont he depth of the contamination. i once had a home with a heated pool that had pressurized piping and used diesel. the contamination went down 40 feet and the consultant went crazy excavating the soil to the tune of $400K!!!

However, soil and groundwater results are below the cleanup criteria in your State.  

Pollution to the ground water could impact neighbor's wells.

I'm on the fence about this one...The REO property I'm interested in is in a subdivision that the ground water is partially contaminated by THC from a manufacturing plant.