The seller has been living there for the past 20yrs and says they decommissioned it when they moved in , but they are unable to bring up the records.

there are state &/or local statutes which require abandoned wells, etc. be de-commissioned, i.e., filled & capped.

When these companies merged, they would sometimes decommission their equipment on a redundant tower. 

They had it decommissioned with papers but plumber said there is still a chance it will be your responsibility after I purchase the house.

(Also Cecil Field which is decommissioned, but a lot of DOD officers work there)If you don't mind 2/1, you can occasionally get a deal in the less desirable side of Murray Hill.

I've since decommissioned the LLC, but am glad I went through the process. 

There is a shared common driveway with an apparent out of service/decommissioned oil tank.

A lot of that will depend on your state's particular laws - both on how a non-leaking tank is to be decommissioned and how a leaking tank is handled.If there is a leak, now is the time to find out, and possibly back out of the deal, even if you sacrifice your EMD.Here in MA, someone who buys a contaminated property acquires the responsibility for cleanup. 

An older decommissioning can mean that there are not many records of what was done and very little may have been done other than just not using it any more, but that may be ok still.

Brandon's first BiggerPockets webinar is about a 5plex that 1 unit was decommissioned so it was a 4 plex to make selling it more accessible to a larger non commercial market.