In Excel you have to really be cautious of formula errors, good luck.

Stay vigilant about unusual payment methods: Be cautious when you encounter requests for payments through unconventional or unfamiliar channels.3.)

What are some liability issues I should be cautious about?

I like to know that my tenants will stay and not be yanked off property by some legal mandate, so absolutely agree with you on being cautious renting to illegal immigrants from that perspective. 

However, be cautious since most local municipalities do not allow short term rentals.  

Some hosts are more cautious about accepting bookings from local people for a variety of reasons.

I would be extremely cautious. 2063 sounds far away, but it will be here before you know it. 

As a generality, I'd be more cautious buying an STR right now as there needs to be a small supply shake out, something that's not likely to fully run it's course for at least another year.  

While this is a GREAT model and we do it with architects in specific development deals (mostly our affordable housing projects to keep pre-deal costs down until construction) I would be VERY cautious if I were you in how much of your time and money you're willing to put at risk for a deal that would have to run perfectly to have a positive/breakeven outcome. 

The past four deals we did we worked with honest sellers and hadn't been burned so we weren't cautious enough.We had used a residential inspector on our past deals, he wanted $3,500 to do the inspection but the seller's agent said he had a good guy that would charge us less that focuses on apartment complexes.