CA has the highest sales tax rate in the country.

The rate will also be high. 

Also I've heard of some DSCR loans that are not too far off from conventional rates.  

As Vince said it is a quick guideline and won't work in most places with current interest rates and house prices. 

Quote from @John Warren: @Mordechai Chaimovitz cap rates and NOI are commercial real estate terms, and 1-4 units are not really valued in the same way.

I am seeing interest rates start in the mid 6% range for foreign nationals here in Florida.

@Tom Rairdon If you’ve owned your short-term rental (STR) for over a year, it likely qualifies for long-term capital gains (LTCG) treatment, taxed at lower rates (0%-20%) compared to your 33% ordinary income bracket.

If they have a good interest rate it might be worth looking to see if you could assume their loan to take advantage of the lower rate.  

Question: Should I collateralize the loan (if using private or hard money), or rely on roll-over loans (to maximize the lower points and 3-4% rate)?

The cost of labor , materials , property taxes , inflation , interest rates  etc .