This business model included 100% mortgages and house-hacking-flipping five properties on the island of Maui.

Essentially, since a HELOC works like a credit card, as you pay down the balance, the interest will be less and less, which allows you to pay off the loan sooner.The one thing I would be leery of is that most HELOCs are based on Prime Rate, also known as the Federal Funds rate, and the rate is adjustable.  

HiI lived in this condo for five years before moving out about one year ago, and then rented it out to a tenant.

I have two mortgages one credit card and no autos loans.

Average annual gross sales of business is $105,000, 26% net ratios, largest expense is debt service on credit cards.

Between HELOCs and/or 21 month interest free credit cards, there are many ways to fund, at least, the materials and just pay cash for the labor.Like said previously, hot water heaters last fooooorever.  

It's a property she's been having for five years, which has remained vacant ever since.

In other words, you don't want a five year loan if the plan is to hold for 20 years. 

Also we visit any late-payers, properties with complaints or city/HOA violations, and every neighbor has one of my cards.

Handing out business cards, or the like?