In this context I'm interpreting killer to mean exceedingly rare or difficult to find.

The big risk is a prolonged vacancy combined with unexpected expenses.  

that's a very good questionmy answer is many of the metric's folks use to decide what a good deal is and what is not, are very regionalized... whats a great metric in one market may not even be remotely possible in another.perfect example is the west coast buy and hold investor pulling their hair out trying to find a 2% deal.. and realizing they either don't exist or are so rare that you would be super lucky to find one.And those in other markets were the 2% rule props are on every block literally and they won't buy them unless they are at least that or much higher in some instances.So this puts investors in a paralysis mode not being able to buy ...

"Applied Accurate Knowledge is Power"You can have knowledge that is incorrect, correct knowledge that you never implement, or a combination of the two.

We told him that we just needed to hear from him, let us know what was going on and we would support him, and rarely (or never) got a response until he paid.

Since it's ideal to approve a rental applicant with income 3 times the rent, I wonder if combined income of multiple applicants over 18 who will be living in the unit qualifies this requirement? 

As far as tenants you get the leases, ledgers can ask for application packages (rarely provided) and the estoppel's but from there you do nothing on the tenant side until you are the owner. 

Non-US persons don't get the (as of 2021) 11M combined gift/estate tax exemption like US persons do.From what I've seen it certainly complicates things by doing it this way and gets really messy when it's time to sell and then you have to figure out who needs to pay the tax on any capital gain/how to file all that.Hope this is somewhat helpful

(And believe me, personally I'm all for young people renting vs. buying, but that my selfish interest)A young couple who have a combine income of $45,000 annually .

Condos rarely are, thanks to the HOAs. 50% rule would give you $400-425 a month in NOI.