His long term tenant recently moved out and is about to rent it to his adult son, who will be paying him enough to cover the monthly expenses.  

@Kristen L GarnerWould it be possible to get a personal loan to purchase a short term rental?

A few I found some information about (mainly to stay away from them as they changed the rates/terms at the last minute), and one had a bunch of fake reviews on the BBB site, but the others I cannot find any information about as far as customer satisfaction.

There are way better options in Florida where prices are dwindling and things are beginning to sit on the market, but the market still supports short-term or mid-term rentals.

Is it better to do an extension of term of the lease or do a whole new lease? 

He's a driver so also stays in hotels often and isn't at home as much as other people.One benefit of me renting to him is that I was going to leave most of my stuff in the unit to rent furnished and will grab what I am ready to take slowly.

Make sure you are asking about all the terms - the maturity date, the adjustment period, the costs, etc.  

I have built myself a BRRR calculator, and the numbers (in terms of cash-flow) are convincing.

I need to open two checking accounts for a couple of SDIRAs and discussing commercial RE loan terms. 

Also, shouldn't the offer of 5% for a 1st time, short-term, hard-money loan between 2 parties who've never even worked together before have been a huge red-flag?