The biggest challenge is to find one to purchase at a reasonable cost. 

Once you find this range you should be very close to understanding your buy box. 

Relationship Building: How will you get to know the borrower?

It's a little ways away from Charleston, which has kept it off the radar, but it's starting to become a more significant beach destination.

We have several hundred keys that we have to inventory and manage.

@Chani Walker can guarantee you can find someone cheaper, but then which services do you want to give up and which do you want to accept less service on?

The BRRRR method (Buy, Rehab, Rent, Refinance, Repeat) could also be a great strategy to recycle your capital.To find deals, try direct mail campaigns targeting absentee owners or distressed properties, or go driving for dollars in neighborhoods with strong rental potential.

Goal would be to acquire 20 rentals by the age of retirement, with some already starting to be paid off.I am in the same boat as you, and I focus on safe appreciation all day (small cash flow for safety and then focused on markets on that appreciate). 

More of a general trend, when we started we asked the same question. how low can I get it?

I recommend you search for "tenant application screening" to find a few different options and see which one will work best for you.Here's some generic screening advice.