So instead of the mortgage payments being on the top of the equation, they will put the income from the property in the income (bottom) of the equation and it helps you qualify to borrow more money.  

If your buyer "might be" this and "might be" that, then take the "might" out of equation.

What you did catch me off guard with was your FMRR equation.

And rent loss and capital costs are part of the 50% as well...Try watching this very unprofessional video I put together that will explain it better:http://www.biggerpockets.com/forums/311/topics/72246-the-50-rule-video-tutorialMortgage payment (principal and interest @4.75%) = $652That's the other important part of the equation for doing the rough analysis...If your gross rents are $1200/month, the 50% rule says that, long-term, your NOI will be somewhere in the vicinity of $600 (50% of gross rents).

As well as there are tons of areas in the USA, mostly coastal or prime downtownish and or university style that can be equated.

In either case, finding the property is the hardest step in the equation.  

So 10 units equates to 10 separate deals and 10 separate loans.

Maintenance costs will make or break the deal, and it looks like you are leaving that out of the equation.

Is it my only factor in the whole equation?  

Part of the equation is why do you want to seller finance while anticipating selling the note instead of waiting for a qualified buyer.