My method has always been to call every listing that hit 90 days on market in areas that I want to buy.

Each property has two savings accounts - security deposit & reserves, and one checking - mortgage payments and utilities 

The main issue I have is contemplating the baloon payment at the end of the 2nd year and refinancing the remainder with a bank...how can I compute that into calculating the IRR?

the rest of the loan payment right now is coming back to you eventually. 

By the time I reach 20% equity (likely 3+ years away), my monthly mortgage payment will come down when I refinance, but $600 is a big hit to the numbers. 

1099, social security, trust payments, annuities, etc?

You definitely want to use a thorough screening method like rentprep.com that gets their income, credit, background check, and call at least 3 references (make sure to verify them by address/job and then their name).  

Keep in mind of the I/O(payments/period) and game plan on how fast you'd like to pay it down. 

So there payments are $200 - $400 dollars under market.

What's the highest success percentage method to find a previous tenant's new address for this?