How does a retiree derive income from their portfolio?  

Unless there are tons of Bear Sterns & Lehman's hiding in the shadows with derivatives and subprime mortgage packages to drop on our heads?

How did you derive $160/month? 

In all seriousness though, if you A) can easily afford the payments and B) will be able to derive a significant amount of happiness from it, then it is probably a good decision. 

@Wade KulesaI believe Dave Ramsey's 10 year number is derived from the common real estate cycle, as 10 years is the time to complete a real estate cycle. 

Explain your formulasNot my formulas, I'm not nearly clever enough to derive them myself, but I'd be happy to point you to my sources Joe. 

He taught the principals of leverage, both with money and multi-units and for that I can only assume the posters are financially maybe a) unaware of the value and haven't purchased his materials, or b) just naysayers because of all of the people who came after, or c) didn't have perspective on what value they might derive from the information kit.I for one believe the information was spot on, and perhaps even too difficult for most to comprehend and put into play, because it was outside the box thinking and truely is hard to pull off with the square-peg round hole RE agents, sellers and financial institutions that we label conventional."

I recently took very early retirement from working on Wall St. in NYC and I have a master's degree in FE so I am very much tune into the financial markets (equity, FI, derivatives, etc). 

Securities and derivative trading not so much but more so since the advent of online forums and platforms. 

Then have an attorney form the LLC and draft an operating agreement if you intend to use an LLC (sole member would have to be the SDIRA with you as the manager deriving no income personally).