In davidson county most people book an STR to be close to downtown or at least close to somewhere cool as a secondary priority (like 5 points or 12 south for example).If you have an STR in white house it's going to need to make up for location.

Are you located in a metro area?

I know that the general guidance is that emergency funds aren't to be invested, but given our financial security and the fact that we already have a personal e-fund in a safe, non-volatile location, would it still be crazy/unwise to invest this 15k in an S&P500 index fund in a brokerage? 

Quote from @Stanley Yeldell: Hi Scott,It sounds like you're in a great location for mid-term rentals!

.- Property Class will affect location, tenants, maintenance, vacancy, etc.2) Not doing enough of their own due diligence when buying a property. - Trusting agents & PMCs too much!

Recognize if you are relying on non enforcement of rules, you are taking on a risk (investing is largely about calculated risks).Enforcement may vary by location but should not. 

House is good, locational significance is not.

So, if you fail to apply the correct assumptions to a property, your expectations won’t be met and it may even be a financial disaster.We use the following to rank Property Classes, in order of importance:Property Tenant Pool: closely linked to location, but not always.Property Location: closely linked to tenant pool, but not always.Property Condition & Amenities: it’s important to, “Maintain to the Neighborhood.”Key metrics for each Property Class:Class A Properties:Tenant Pool: Majority of FICO scores 680+, no convictions/evictions in last 7 years.

I lived in it for a year and loved the location.

Little Elm and most cities in DFW are not walkable location so they need a car to move around.Many things to consider, and there are only a few that I know that have done it in a short scale.Happy to connect and rainstorm more.