This is probably a more complicated measure of seller financing because the real estate can be destroyed and the buyer would back out or the assets would appreciate significantly and the seller would request more money.In the above case - I guess the buyer could buy the rights to the income from the seller until he brought out 100% of the LLC.50 is still a young age - I was thinking they would benefit from deferring the gain as a result of inheritance and getting a step up of basis for their heirs.Sorry if I misunderstood your LLC scenario.

I would first define what goals you want and what metrics you want to measure.

How would you measure/analyze that deal i.e. what factors would make it a winner for you and make you pull the trigger?"

The REO stigma is a measurable value that banks recognize as a discount seen in the market once a lender has taken the property back. 

What are some added security measures that you have or have heard of someone putting in their contracts?

(There is an acronym which I learned it is called SMART which means: Specific, Measurable, Attainable, Realistic, Timely) Look it up.3. 

I can install Google Analytics to measure how much traffic my website is generating, which allows me to then install a code for retargetting ads that will follow that site visitor.

You should be allowed to have your contractor do a walkthrough and at least measure and estimate for this worst case scenario so you can determine if its still worth the effort.

Contracts with deadlines with positive and negative measures for finishing early or late

To piggy back off sticking with 4 units or less which will allow you to put less money down for a residential loan vs commercial loan there are added benefits when it comes to cash on cash returns - which in my opinion is a great way to measure an investment.