Besindes replacing the old leaky tank, you still need to do the following:
(1) Do soil tests to determine the extent of the contamination around/below the tank.
(2) Remove the contaminated soil with clean backfill.
(3) Do soil tests again to confirm that it is completely cleaned up.
(4) The soil testing agency should provide you with a signed letter to document the clean up effort and test results.
I think the clean up is usually several thousand $'s, unless the contamination is realy widespread. In any case, I would suggest you do the first soil tests to see how bad the problem is. Make this a contingency in your offer, so you can back out of the deal if the tests indicate a huge contamination problem. Soil tests cost several hundred $'s.