I have a note in the lease that they're not supplied or warranted by the landlord and the extent of what I will do (if any exist at all) is haul them away. 

Promissory notes in general, as they have no collateral, seem a bit higher risk, perhaps mortgage note investing which are backed by residential and sometimes even commercial real estate would be a safer alternative, and yet very lucrative, too,  8-12% yieldsFor extra returns, I put a small portion of my portfolio into BDCs, Business development companies, that specialize in loaning money to growing businesses which are too high risk to get loans from proper banks.

Although we’re not a local bank or credit union, we are independent mortgage brokers with experience in financing options that consider rental income rather than personal income. 

If the rate is reasonable then it may make sense to try to save fast to get to the 3.5% down payment on a double or triple - househacking may allow you to increase your savings rate and pay down that loan even faster.As for estimating costs, closing in NY factor at least 1% of the mortgage amount for tax, plus $1500-2000 for lawyer, title and fees, then the amount required by the bank for escrow will depend on the actual property taxes and insurance.

I was shown about a handful of their holdings in the Houston, Memphis, and Little Rock markets, but the cash flow and CoC returns were less than $100 per month and 2-4% after factoring in additional expenses like 7% vacancy and 10% maintenance per month to go along with 20% down and conventional mortgage.

This amounts to lost opportunity because if you had purchased RE, at the closing it can start producing return. 5) ADUs detract from the existing structure whether this is privacy, a garage, or just yard space. 6) this is related to number 1, but there are many more buyers looking to purchase homes for their family than there are RE investors looking to purchase small unit count properties.

Moreover, can you clarify if the complex is already an existing 16-unit or do you intend to build it?   

Mortgage is only less than $1400.

Deductible expenses include a proportional share of mortgage interest, property taxes, insurance, utilities, and repairs.

I have a mortgage connected that, for some reason, reports it as an escrow balance, throwing off my balance sheet.