The owner is open to seller finance for us.

I’ve considered the following options:1.

How should I go about financing this?

Before moving forward, negotiate better terms with the hard money lender you initially spoke with and ensure the seller is fully on board with second position financing.

Rehab estimated at $35K3 unit propertyWill need some financing What made you interested in investing in this type of deal?

Great question - 20% down is the typical down payment for investment properties, regardless if it's a single family, duplex, tri or quad.We have a 15% down option for the same type of loan.

The main condition was I had to have the downpayment (20%) first, close the deal, and then execute the owner financed 2nd mortgage. 

This will affect things like required down payment, rates, and even which lending institutions would be willing to finance. i encourage you to look up warrantability for financing just so that you don't have surprisses when your 20% down buyer cannot finance that way.

I have an opportunity to purchase a mixed use property that seller is open to using partial seller finance.

I have originated both conventional and DSCR types of financing.