Now, not to mention the interest rate on margin is always a lot more than a mortgage rate.

A good mix of LTR'S is a very good idea so that all your eggs aren't in the STR basket.I like tourist destinations where STR'S are heavily ingrained and almost impossible to ban or regulate out of existence.

Conventional mortgage product? 

@Kris TohovitisI wouldn't recommend using your cash to pay off the mortgage on your primary residence since you can likely get a better return on that money by investing it.What's your buying criteria for the multi-family property?

People like to use HELOCS especially now, if they have a very low interest rate first position mortgage, in order to tap into that equity without losing the low interest.

That is if purchasing EXISTING performing investment real estate AND have very good clarity of every facet of the transaction ESPECIALLY reasoning for seller selling, AND you have decent market intelligence. 

Mortgage, HELOC costs and terms, etc.

Specifically, I am looking at partnering with an individual who owns securities (stock and mutual funds) having them take out a line of credit against their portfolio (SBLOC) at a low interest rate, and using that to purchase properties rather than using a mortgage.

For full protection, your mortgage company will likely still require you to carry a landlord or dwelling policy.

Having said that, we also have many clients we use our systems and processes for crypto funds, buying existing businesses, funding startups, hedge funds, venture capital, and even non-profits.