I'm looking to purchase my first property with my fiancé (both 25) with the goal being to establish our primary residence together, as well as giving me the opportunity to add equity/value via rehab before moving on and converting the property into a rental within a year or so to kickstart an investment portfolio.An interesting lead that recently came up is my parent's neighbor's house.

You could do something like what you want if it was your personal primary residence and even then the interest rate would be ugly. 

I'm seeing house hack, buy a primary and rent out the ADU on AirBnb or the ultimate house hack, owner lives in the tiny ADU and AirBnb both levels of the main house in San Francisco. 

We got both primary auto and umbrella insurance on it.  

Why not just buy a new primary and rent out your current home.

It would be his primary residence so most likely a duplex or triplex house hack. 

We own our primary home in the Portland area, and have had an STR condo in Seaside OR for 2 years.

I talked to my lender contact and, as Kerry mentioned above, it's pretty uncommon that a traditional lender would offer a HELOC product on an investment property (non-primary home). 

It’s great to have a community to bounce ideas off of.

Reason why is because FDIC Bankers "Do Not" charge points on traditional programs like primary homes, standard investment purchase and refinance, Second home/vacation homes.Lenders, Brokers all charge points to meet their Loan officer compensation plan (commission).