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.

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.

Do not waste your money on his course.  

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

. $40-$50k out of pocket, HELOC or private money loan for the remaining $20-$30k 

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.  

depends on your credit if you can get a mortgage or your ability to find good deals that you use hard money for and then refinance into DSRC.  

Its not my money.