An engineer or a health official may take randome soil samples and provide a report.

I thought about filling it about half way with rocks, then adding soil, and having a "sunken" yard.

Oil prices dropped in half in the year 1986 alone, along with a 6-year price drop due to the 80's oil crisis.

As far as the tree, it depends on how close it is to the house and what kind of soil you have.

@Daniel Moctezuma, I don't have any direct experience but the problem that I see is if the tank leaked you may have an environmental clean up on your hands (need to clean up the soil as well) - it may not be as "simple" as only removing the tank. 

Is you need to put money aside for foundation issues its not if Its when.. everyone of my homes I had we had some sort of work to do to address shifting soils..

Lending guidelines exist for a reason.Please bear in mind that we're just now coming out of a very bad recession because the banks handed out loans to people that they really should not have in order to sell the bundled loans.  

Once you had a vacancy, showing the unit would be a bear.

Texas is very tough for a CA investor.. sky high property tax's  soil and foundation issues bad hail storms.. and your income taxed in CA.. so for me its a value add sell market.. make money but do not hold in texas.. but even that only the locals do very well at it.. its a tough place. you need the non income tax to balance the property tax's and other issues.

If there is a hazmat spill from an UST they bank won’t underwrite the loan w/o remediating the soil especially if near wetland.