How to properly buy our house from the landlord without mortgage

[Maxym Le Deux]

Hello everyone! 

We really need a little guidance here to go in the right direction. Long story short, we are buying our landlord's house. It's a 2 story duplex occupied by the landlord and us. We have a long-lasting relationship and have agreed on all aspects of the purchase. The landlord still has a mortgage and the house is significantly below market value. The idea is to put a good down payment and increase our monthly payments to the landlord so it covers the mortgage and more that will go towards the overall purchase price.

Since there is a mortgage I would like to avoid triggering "due on sale" and hopefully a reassessment of the property (if that is all possible or at least delay it). We are in CA and not sure what type of contract is best for this situation. I'm thinking a blend of owner financing and rent to own of some kind.

Rent to own (con) - we are not planning to go through a bank later and we are looking to buy now
Seller financed (con) - possibly cant take ownership since there is mortgage 

What type of structure (or at least direction) can we possibly take to purchase the house? We don't mind if the landlord retains the deed until all payments are made but we would like some protection. What kind of contract can we do and what kind of protection measures can we implement?