The mortgage is secured by the real estate.  

Best approach is to have your "team" set, before going out to secure properties. 

With your strong credit score, high income, and solid investment portfolio, you are in a great position to secure financing for an investment property in the U.S., and as they've mentioned above DSCR is something you might want to look at.

What is the security on the above?

It does sound to me that a lot of that work should have come out of the tenants security deposit 

I understand this is a good way to become financially secure over time and am eager to start.  

What I love about them is they will even accept the higher lease amounts over the market rents if we can show first month's rent and security deposit.

It is the interest rates at which banks lend to each other and is short for Secured Overnight Financing Rate. 

Now, over a year and a half later, he sent a certified letter last week demanding the security deposit or legal action within 30 days.Please advise. 

Leveraging our market connections, we identified the property’s potential and negotiated favorable terms directly with the seller, securing it at a highly competitive price.