Now how that all actually plays out with insurance, attorneys and courts in individual cases, and at what costs in money and time, that's the rub.There is usually an insurance policy on the tank, to cover leaks, as part of the price of the fuel, so check with the seller on that.I would certainly recommend making your deal contingent on a clean leak test (and decommissioning if that's what you choose) and have your deal effectively read such that if the test comes back with water in the tank, ie the tank has leaked, then you can walk from the deal and get your EM back.The existence of a UST, especially if in use and not leaking, in no way needs to be a required deal killer though. 

Best policy going forward...no good deed ever goes unpunished.

I find a combination of it being much easier to buy (30 settlements vs 1 settlement), manage (30 different addresses vs 1 building, 30 insurance policies vs 1 policy, 30 spring roof inspections vs 1 roof...), and yes, economies of scale ($850/yr insurance on my SFH vs $7k on the building) makes Multis more attractive to me than all SFHs.  

Or is their still some sort of "public policy" or law that would put them on the hook for this?

My why's are my own cash flow analysis and my own break-even point for a 15% CoCROI with a 5% downpayment and some safety margins.

The most I can say on this deal is that I think there is a margin of safety in the purchase given the numbers presented.

The key to any type of investing is to educate yourself as much as possible, give yourself as much of a safety cushion as you can, proceed with caution, use good ole' common sense, and get started as quickly as you can.

The issue I see is that you are essentially paying retail for the lot and not leaving yourself with a margin of safety.  

Our homeowner's policies are for those two home's as well as our personal home.

It's easier to buy a good umbrella policy (which you should have any how).