I did residential tank cleanups for years. I agree with everyone who said to have the seller pull it. Even if there's a letter saying it was tested or filled with sand. I've pulled a lot of "clean" tanks that had clearly leaked for a long time.

Getting financing probably won't be a problem, getting insurance might not be either. The problem is the basis of environmental law: if you own it, you're responsible, even if you didn't spill it.