Makes it easy to keep the cash organized as well as bills.You don't need a credit card but debit cards for each place makes it easy.

Your credit score needs to stay relatively the same from application to closing. 

If your rate is low on the 5 unit, I might consider the possibility of a line of credit for my equity instead of an entire new mortgage.  

We bought it for $225k with a 3.125% mortgage and 1) are interested in keeping it as our first rental. 2) We are thinking about using some of the equity (maybe $50k or less as a HELOC to combine with existing cash for our next home down payment that we can convert to a HELOAN through our credit union).

Supercharger system is far and away best system out there.. there is no POS charge system its on your credit card and automatically billed.. my first 3 Teslas were grandfathered in and I NEVER paid to charge however all the early buyers still get the free charging.

Most of my clients run their SFRs at a 25%-30% (excluding vacancy/credit loss) operating expense margin and most of the banks I deal with on their DSCR loans underwrite using 35%.

However, I've researched different ways of doing this, such as a home equity loan, Home Equity line of credit, and cash-out refinance.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I ran some quick numbers and would estimate (assuming you have good credit) you could borrow around $415k-$420k, which includes funding 100% of the rehab costs.Have you calculated your expected profit based on selling after rehab and if you decide to keep long term? 

Also remember just because a loan may qualify as being assumable the lender will go through a thorough underwriting process to confirm you meet eligibility for the loan.I have seen a guru teaching people with poor credit to go after these types of loans and it’s unfortunate as many would most likely not qualify.