I would like to purchase another property so I looked into a heloc and a cash out refinance and it looks like I would qualify for about 75000 max which would be enough to put a down payment on another property and rehab it, but I still haven’t paid myself back from the original property rehab.

Its great that the seller is will to carry a note in a seller 2nd as well, that will help reduce your require monthly payment with the right lenders.

The rules and payment standards vary significantly from one area to another, so familiarizing yourself with the local housing authority's procedures is key.

I offer an online payment portal to submit rent payments via ACH or CC - they "couldn't figure out how to use it" so I set us up to accept Zelle payments.

As long as the rents can cover both the current PITI and proposed P&I payment, credit is at least 680. you should be able to qualify. 

Unlike the Bay Area, at least from my experience in Sacramento, i don't think the difference in down-payment from a duplex and tri-plex is that significant.

I still remember the first loan payment coming due and wondering where that money was going to come from!

I assume you currently have a residential mortgage that does not have a pre-payment penalty.

Quote from @Jack Townsend: I heard David Greene mention a loan product that only requires a 10% down payment on the purchase price and 10% of the rehab budget.

If they don't NEED the money for a down payment elsewhere, the income could certainly fund their payment at a new home, rental or retirement home.