Private sewer improvements can easily cost $500,000 or more. 

Seller Financing plus private party loan How did you add value to the deal?

Why not just seek to borrow money from this person as a private mortgage secured by the real estate?

You can do that by borrowing privately from friends, family, and family of friends.

If I have a partner with private money willing to put up the down payment, how do we structure the deal?

PMI, the private version, on a conventional loan will generally be cheaper assuming you qualify. 

Is it a straight play to cut out the competition from private equity so everything goes to the huge investment firms that are left?

Shoot me a private message and I would be glad to share. 

Although, the private side of a bank may help. 

My intent is to use the line to either be able to buy quickly with cash or use the line *** down payments and rehab money on fix and flips or for properties I would buy with private money and then refi after rented.