I have some questions (and they might be silly, so bear with me.1.

He says that in the unlikely event there has been some leakage, he will "dig out some of the soil and replace it."

What state you put it in really has no bearing or effect on its effectiveness.No legal advice given

Even if it was "properly" abandoned years ago, the possibility remains of soil contamination below the tank.

Jim probably has BEARS for tenants.  

Ideally take out with a loan once rehabbed based on rehab value (I need to understand what the market would bear for rehab value - probably not a huge increase once rehabbed - value is in the Airbnb income).  

I am looking at Sugar Land , but the market is rather high and Texas property taxes are a bear.  

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The first thing is to make sure the soil slopes away from the home and gutter/downspouts are in working order.Other than having a good working sump pump, other methods I know can be hit or miss in effectiveness.Some people have had good luck installing a french drain system in the yard to carry away excess water to the low point of the lot.

I wrote into my lease that rents are expected to rise 3 - 5% annually just as an FYI to my tenants.Then I can come back at lease renewal and be a good guy with "only" a 2% rent increase.As Tod suggested though, it's really what your market will bear.