Which type of loan works best for this fix and flip strategy?

[Nathan Paisley]

I have a married couple who has roughly 700k in a free and clear home that they live in. It's in an LLC, the first two floors are rented out, and they live on the third floor. 1 electric meter. They want to pull out the equity to use for a fix-and-flip. I'm going to partner up with them. They finance the deal, I fix it up, and split profits 50/50. I'm a licensed GC who has flipped over 40 sfh over the years. At the moment, my main business is home remodels, i.e., kitchens and bathrooms but I want to flip full time.

I'm looking for the best strategy for them to A, pull out as much as possible or as much as needed when needed. B, not get crushed on taxes/fees and C, how to rinse and repeat if all goes well. 

Hopefully, I have given enough context of the situation. Also, if anybody has any pointers on structuring the deal, I would love some of your insight!

HEL, Heloc, DSCR?

Thank you so much!

Nathan