Without an exact numbers breakdown of each option (which should obviously be my next step), I think my ideal situation would be to sell the SF and roll the equity into this, leaving me with a modest holding cost that I can absorb with my stable W2, and keeping the HELOC for any larger ticket items I need during rehab.

I guess I have to talk to the owner to make his offer better than banks, I feel like he is not motivated enough, he said he wanted to retire. 

It's not going to retire you in 5 year but give it 10 - 15 and you're likely sitting pretty.That said, it's not as easy to pull off as it sounds and not as smooth sailing as folks will make it out to be.

I don't plan on switching without finding the same functionality - I was just hoping it existed in a more stable product already.

I’m married with 4 kids, I make about $175k a year, we have about $250k of total debt, about $500k to $600k in equity in our current property, around $15k in liquid cash and close to $200k in retirement saving.What does action look like for me and my family?

We originally wanted to hold for the long term to potentially retire in but at this point, there may be better use of our money if we can sell. 

First just want to state, she is your girlfriend not your wife, which is very different in regards to financial status/situation.For me its simple, I am buying this asset and taking on the risk of renovating it and as part of it will be seeking to earn income from it to better myself in the future as I know a W2 job and a 401k does not get you far in retirement.

If you ask most small-time NNJ investors today, they are investing for appreciation and are OK with breakeven scenarios on the cash flow side, which is risky even for a stable market like the NYC metro area.

I have a decent amount in retirement accounts but I'm not sure if it's a smart move to leverage those. 

-Current Assets: <$100k in cash, ~$300k in retirement accounts, Primary Residence (CA) ~$100k in equity, 1 LTR condo ~$200k in equity and a HELOC @ $80k to close this week hopefully :).