.- I got used washer/dryer from them.

The buyer definitely owns the property, otherwise it is not owner financing, it is some sort of lease or option or a combination of the two.

If you bought 5 properties with $300,0000, they all need to be throwing off a total combine of 20% return to get that $5,000 (as the example above shows), to get that expense coverage for your ability to leave your job. 

It might be a combination of stocks, bonds, mutual funds and alternative assets.  

I am seeking recommendations for companies or resellers that specialize in App/Card-based Washer/Dryers.

Washroom bring your own washer/dryer(currently).    

If the other houses you own have no loans could you stack all their combined monthly rental income to pay off the remaining $120k mortgage before you retire? 

There are some out there that teach how to best use a combination of marketing types.

We have long term renters in Houston that are a 30 year old married couple, combined household income of about $110k, and they're going to be renters for 3-4 more years (based on their own admission).

that short term loan - whether it's a HELOC or hard money or cash or a combination - has to be paid back regardless of what the end product appraises at.