Financing options for purchasing a 50 SFH portfolio

I'm working on a deal to purchase 50 SFH located in the Midwest. The price is approximately $5.5Million. The current owners have financed the properties with a combination of Fannie Mae loans and portfolio loans from several local banks. They owe approximately $4Million. I will definitely be looking to incorporate some seller financing into the deal. I would appreciate your suggestions on the idea structure and financing options for the first mortgage.