What is the best way to structure this deal?

[Kapil Kandel]

Seller is offering to sell his house. The sell price is lower than the value of house. Seller wants me to take over his mortgage payment as soon as we make the deal. Also, he has given me an option to pay (sell price - remaining mortgage payment) amount after 6 months. I will be using this house as rental property. What will be the best way to structure this offer so that I can either use as rental property or sell after some upgrades? Or is it better to get my own mortgage at the beginning rather than assumable mortgage?