Hi Ashley, fellow New Jerseyan here.
I want to address a couple of your assumptions here, especially as they pertain to the CNJ/SNJ market:
1) Rentals are in huge demand everywhere in NJ. To quote Field of Dreams, "If you build it, they will come". Even in South Jersey, where the purchase prices are lower, there is a silent pool of tenants waiting to be able to afford a place to live that may not qualify for a mortgage. If you want proof of this, look at every Section 8 waiting list in the areas you are considering. I'm not saying that you should consider renting to Section 8, but just to show you that the demand is there even if there are very few MFHs showing up on the market as available for purchase/rent. Always make sure to adequately screen your tenants (background check, income verification, credit report, etc).
2) In the context of House Hacking, with a SFH are you considering renting out by the room? This is a very different way of life than renting out by the unit. Renting by the room would increase your rental income, but you may sacrifice in your own quality of life having to manage so many tenants. If you are comfortable with renting by the room, check the local municipality laws in the areas you are considering to make sure there are no restrictive laws about operating boarding houses. There are a lot of posts on BP about how to navigate rent-by-the-room investments, and these are more commonly allowed in South Jersey than in Central Jersey.
3) House hacking and cash flow do not coexist in NJ in today's market. You can house hack and reduce your expenses, or you can (try to) cash flow by not living in the property. But you can't do both. If your goal is to house-hack short term, and you want to make sure the property will cash flow when you move out, that's understandable and there are ways to do this with forced appreciation or by buying in a higher-risk, up-and-coming market. But house-hacking will cost you money, not make you money. And it will definitely be more than $400/mo!
4) MFH financing has the same product options as SFH financing for owner occupants. You can qualify for FHA loans (3.5% down) for up to 4 units, as of the last time I checked. There are products out there that allow you to use a property's current rents as income if you are acquiring a property that has tenants with leases already in place, and this would help your purchasing power. I would recommend reaching out to a few local loan shops, and highly recommend credit unions in today's rate climate, to see what products are right for you. I do not recommend pulling a home equity loan as a first time investor if you are buying a fixer-upper, the current rate environment is not favorable for rehab projects that run way past their scheduled timeline and you don't want to run into a situation where your family can't meet loan payments while your house is securing the loan.
Hope this helps, I'm happy to share my experiences investing in NJ if you have any questions.
Good luck!