He rejected this, effectively calling me a slimeball for refusing to pay him the $750 outright.Am I out of line here?

If you want to test the effectiveness, buy another domain and put that on the side.  

Manage your property effectively, be especially careful in the area of safety, use common sense, and have a lot of liability insurance.

By disclosed, I mean have her sign something to the effect.  

The following was found in a google search:When septic systems are properly designed, constructed, and maintained, they effectively reduce or eliminate most human health or environmental threats posed by pollutants in household wastewater.

I'm struggling in finding resources on how to effectively and accurately calculate the value of property as well as AVR.

The latter option would save you the out of pocket expense while effectively financing the cost over the period of the loan.  

IMO, it takes a certain personality type to be effective as a property manager on a larger scale.

If you lack knowledge of how your actions effect others you may never see the unintended hardships caused by what you do.

You will make a TON more in the long run this way, but if you're looking to cash out and move your money into another deal this might not be what you're wanting.However, if you use that down payment to turn around and pick up more properties with owner financing you can start a domino effect that could really build up your portfolio!