In the event that a seller was to owner finance a property to a buyer without a SSN, is there a market for the Note to be sold to a note buyer?  

If you really hustle and are smarter than the average bear, you may make enough money that you can buy an investment property.

STR Property management is common and I know a lot of folks doing this.  

I would also add that you join your local REIA chapter (Real Estate Investor Association) as it will allow you to network with other STR owners and get more location-specific information. 

Since part of your property is a rental, I believe a portion of your mortgage interest should be a tax deduction unlike your other debt.

As a former business owner, there are times of the month where it seems that all the cash goes out at once.

90% and 100% Occupied...Direct to owner

In addition to STR'ing the main house, he is building 2 more units (ADUs) on the back side of the property.

In Indianapolis C to B class neighborhoods I have rarely seen insulation upgraded on rentals especially doubles that have never been owner-occupied.  4th air gaps,  poorly fitting windows, and doors can be easily corrected with proper caulk and weatherstripping (usually).

There's actually a huge oversupply of properties in Tulum and none of my friends who own property there make anything close to that!