I have been extremely busy lately... But one of these days, we gotta sit down again. Basic math: buy 5 rental at 20% off market price at roughly same value will give you 1 free and clear unit (5x80%=4x100% LTV plus 1 at 0%). You then can either take 50-70% LTV first position heloc for other 'emergency fund/ down payment/operating cost on the last unit.. Or simply sell the first 4 to get rid of debt and own one free and clear. It doesn't always work out exactly that way, but it works great as a guideline. That's how I got a few free and clear units. But the most important key is still rental cash flow.
On mls search for "portfolio" sale.. You'll see listing of 5-10 houses in a bundle package. Those investors are doing exactly the above.