Whenever someone wants you to sell on an installment plan, they are asking you to be their banker and lender and carry the risk. Then, the buyer intends to work on the property, even though it won't be paid in full for 25 years. Which means the potential risk for damage is ridiculous. This seller could find themselves stuck with damaged property they cannot resell and stiffed for the balance. Even if they get half up front, is that worth the risk? And with those numbers, why can't this buyer get any other lender involved?

Inheritance is the least of the issues in this scenario.

The seller needs to insist of full purchase price, or let the heirs sell it later, and they should know to get a better deal than this, no matter what the selling price is. This deal smells.