That is because I make an additional payment on principal every month on top of the normal mortgage.

On an investment property math is suggesting you take the money you would use to pay off the principal (presumably 75%) and buy 3 more (with 25% down).I always remind people that in 1960 you could have bought a brand new house from a builder (with automatic air conditioning!!!)

Higher principal at better market value w/ no money down and no interest straight ammortization for 20 years) I'm excited because it would be great to have essentially two more units next door, self manage, etc.

When you add in PITI (principal, interest, taxes, insurance) are you going to cash flow on a monthly basis?  

As principal, I hired our construction company to do the build at a fixed profit margin.

Every once in a while I would get a bug on principal so I would turn it into the Section 8 people and they would write a letter.

Full $9mm at 8%= $720,000 per year, even before Principal and income taxes.How will you cash flow the Seller Financing and/or meet their balloon payment in say 3 to 5 years when they want to exit?

If you need the monthly cash flow and have a lot of capital, then you might want to pay down the principal as quickly as possible.

I have no knowledge of the principal or his investment vehicles.  

It is best to receive comparative preliminary analyses (estimates) from providers with construction engineering backgrounds, cost estimating and allocation experience, and knowledge of applicable tax laws who use the detailed engineering approach from actual cost records or cost estimates, and adhere to the 13 Principal Elements of a Quality Cost Segregation Study.