Seller was clear that they wanted to retain mineral rights. 

Some examples of regions that end up in the top 10:Fastest population growth (Jackson County, Georgia)Highest net migration rate (Horry County, South Carolina)Fastest job growth (Eddy County, New Mexico)Fastest income growth (Mineral County, West Virginia)Fastest home price growth (Grays Harbor County, Washington)Fastest rent growth (Franklin County, Massachusetts)Highest rent to price ratio (Jefferson County, Arkansas)Is this kind of data based searching worthwhile? 

I would have the water tested for pressure, minerals, metals and bacteria.

Often those are stripped away long before development similar to mineral rights.  

Some of the research I've done says to look into questions around fencing the property, water/irrigation, minerals for the land. 

Surface, minerals, leasehold royalty and production.

@Charlie Miner Proof of rent payments and outside of that sounds like you have it all covered.

In VA:CONDITION: The owner(s) of the residential real property makes no representations or warranties as to the condition of the real property or any improvements thereon, or with regard to any covenants and restrictions, or any conveyances of mineral rights, as may be recorded among the land records affecting the real property or any improvements thereon, and purchasers are advised to exercise whatever due diligence a particular purchaser deems necessary including obtaining a home inspection, as defined in § 54.1-500, a mold assessment conducted by a business that follows the guidelines provided by the U.S.

Or you can go a different route and move into oil gas and mineral resource royalties via the 1031 as well. 

Working with the city to ensure proper zoning, land use, any mineral/water rights ect to name a few.