Proper contract type for owner financing?

Hello everyone! 

We would like to buy our house from the landlord. Currently, it's a duplex with the landlord occupying half and we the other. We already agreed to buy it with "owner financing". Since the owner still has a mortgage on it we cant get our name on the title without triggering "due on sale" The agreed-upon deal is to provide a good down payment, pay extra every month (will go towards the sale price) + expensive home improvements. I don't mind not having the deed until its all paid but would like the deal to be protected from the following
1) owner backing out in the future (especially after all the home improvements and payments)
2) Relatives claiming the property if the owner dies (no children, no spouse but does have a sister and knees)

What documents can we create to protect not only the owner's interest but also our own investment? How would it look like? Should there be a trust or just a modified owner finance contract? Should we have a trustee?