I initially offered $200K in cash, but the seller countered at $240K.

We self-manage our five-unit property with both—while RentRedi's reports can be clunky, Stessa offers much better reporting.

Also have made it easy to see if you offered a certain amount, or changed your rehab budget, how that would affect your return. 

I wish they were offering some uses of it. 

Unfortunately, NY does not offer many local STR companies due to the regulations that make STR hosting difficult. 

What price would you offer?

You can offer whatever you think is appropriate and message the seller with your reasoning.

They have failed to renew my entities with the Secretaries of State multiple times, have billed me wrong amounts, and GREATEST OF ALL, have cost my business ~$50K in cumulative TAX PENALTIES due to negligence in filing for a tax extension, failing to walk me through my tax filings and needed payments, and failing to file an abatement promptly.

I am guessing only you would be signing the 18 month lease and this individual who is there to "help" won't be voluntarily offering their signature as a co-signer?

You are going to need to see their tax returns to see if they are telling the truth.And what is the interest rate that they are offering you?