First, they called Property Manager and claimed gas leak. 

The warnings are on most buildings, restaurants, markets, gas stations, auto dealers, wherever alcohol/tobacco/Rx are sold, etc.

I’m talking through this… Since the lender will be aware of the title change the existing insurance in the sellers name will no longer be valid in which I will need to have a policy in place under the LLC I’m buying under.

Currently we have a 50 gallon gas heater.

https://smallbusiness.findlaw.com/business-contracts-forms/what-is-specific-performance-as-a-legal-remedy.htmlThat will take time and expense, but if the contract was valid, fair and equitable, the court can compel the sellers to follow through with the sale at the agreed upon terms.I don't know whether you can include legal fees as part of your claim, but I think it's worth looking into. 

25 properties all SFR 30k each all occupied tenants pay gas and electric.

Cheap units don't have ecm blower motors and two stage gas gas valves. most of the big names also own other cheaper brands that are similar but with less bells and whistles. 

Am I being paranoid or are the concerns valid?

@Ronald Rohde Valid advice.

If your relative signed the contract, it's a valid contract absent some clause in it that allows him to cancel it.