I personally don’t like installment sale contracts or contracts for deeds, especially over long periods of time.

They suggested adding an external cleanout for about $8,000, and we agreed to split the cost by adjusting the home’s price.However, after installing the cleanout, we discovered that the actual problem was far more severe.

They are basing it off a depreciated cost from an original install price of $1440 and claiming a 7 year life cycle.

But they did so with the request of the seller buying and installing new washer and dryer in the house. 

In my county we have a homestead tax credit for owner occupied houses which limits your property tax increase AND you can pay your taxes in 2 installments .

We've also privately lent with similar terms for our new installs.

Significant defects in the roof and siding resulting from improper handling of materials, and improper installation. 

Who is paying for the installation?

You could start an HVAC company that does installs and/or does trouble shooting when it's peak season.

Its not that far off for two, and a heater, plus installation with a little price gouging.