I don't have experience with this exact scenario, but with my recent interactions with the city Zoning office about a case I'd venture to guess that since you are not trying to change the use, but rather are trying to reduce the overall number of units, you'd not need to re-zone.  

I am just recently joining the platform as a CPA and looking to answer any questions you might have regarding tax accounting for your real estate ventures, or just general accounting questions!

Assuming you're looking to do a buy and hold: one of the simpler structures is a Joint Venture where your money partner buys the property in their name and they pay you like a contractor and issue you a 1099 at the end of the year.

Of course a joint venture is one approach but I also have a house with a lot of equity and was wondering what your experience was with using that equity to start your fix and flip or long-term rental business?

If equity, your best bet is to create an entity (usually an LLC) to register each party's equity interest in the venture. 

At least that's what I would do if you're running this as a joint venture. 

We decided to venture into real estate about 9 months after this purchase and a recent refinance due to way lower interest rates.

Stocks, real estate and venture capital.

Good luck in all your ventures!

And are there any private lenders that are interested in this type of venture?