This could potentially meet your financial needs by itself as well as provide start up money to do flips or purchase buy and holds

Invest your money in a better deal. 

Consider financing options like hard money loans, or FHA loans for house hacking, and focus on areas with strong rental demand near the MA/NH border.

Selling it now vs renting, will give your dad more money and he won't have to worry about renters or cap gains. 

I would be less concerned about the cheapest money and more concerned about decent terms and execution.

These are some of the most successful people; CEO’s, tycoons, celebs, old and new money and investors on the planet.

If you just want to build your cash reserves then flipping is the way but just be careful - that too takes a lot of work and research to make sure you don’t lose money.  

Its not my money. 

In fact, I would estimate that it is mostly upper middle-class folks that have the money and wherewithal to go through the vacation rental process - prepayment of substantial monies, etc.If you only have one property, I would just stick with Venmo, CashApp, or Paypal.

In most cases you will loss money (in the current year) if you need to turn a unit for a 10% rent increase.