what’s the best way to use OPM to buy my first house to rent out?

I am retired and working on career #2 and 2 of 3 of our kids have left for college - our oldest is headed to UConn for her PhD (6 years) and instead of paying to rent, our thoughts are to do a version of a house hack with buying a house/condo and renting out one to two rooms to offset the costs of her rent.

What are some companies that buy these loans?

So don't buy rental but keep buying primary locally... you can even double your networth very quickly buy buying broken-home and flip it around. 

My brother and I have sent out direct mailers, very generic we buys houses cash with a phone number and a we buy house sign for the picture.

What are the disadvantages of buying a 2-story duplex? 

(In other words, if it's too high, then I can cancel the buy and not have to lose any money)2.

For borrowers or investors interested in properties that sellers will not carry or without an assumable loan or accessible reserves, my suggestions would be to try and secure a seller concession towards closings costs or an interest rate buy-down.

That means in 21 years I will have a fully paid off property, and can now cash out refi and buy X more properties.

Investment Info:Single-family residence buy & hold investment.