It may be more economically feasible to drill a water well $3,000 & install a large septic system & field line $3,000+/- if the soil will properly perk.   

If the wall is on the main level, look for supports underneath it.

Whoever you use, make sure they are getting up on top and underneath (if applicable) the house you are purchasing.

As well as do things like soil engineering reports and archeologist studies. 

I have another investor friend who owns unbelievable amounts of real estate from the ground underneath international franchises at major intersections to mobile home parks with hundreds of units.

Saying hey, I found an oil spot and the neighbors say... isn't going to carry much weight.If you do get (and maybe even pay for) and inspection and estimate to support your claim, then be prepared to remediate the problem as well, since the property is selling as-is.The upside is an estimate will be worst case, and you might actually spend much less to dig up and replace the soil.

The way I see it on the surface, I'd be better off buying a $400k (ARV) house at 75-80% ARV than a $160k house at 70% (ARV); I'd have a more desirable house with a larger net profit, and if things went sour during the rehab I'd be able to get out from underneath it easier, being that that type of home is more in demand (therefore less risk in that respect).

Even if you do plan on tearing a property down and rebuilding, you still ought to take a look at the land, soil and sewage.

The carpets are heavily soiled and the walls are marked throughout. 

I was concerned about the cliff erosion which is an issue in Southern MD so I spoke to the local soil conservation service.