If I understand correctly, you are looking to buy a home where the owner is providing the financing and your concern is that after you do a bunch of work they will take the home back by foreclosing.

Keep in mind that the only way they can legally foreclose is if you breach the terms of the loan - if you make the payments and otherwise do what you agreed, they can't take the home.

As such the primary thing you need to worry about is making sure you will never be in a position where you are in default (didn't do what you agreed to do). On that front I would have an attorney carefully review the note and deed of trust (the documents that make up the loan), and explain to you each of your obligations. Make sure you are comfortable with those obligations. Specifically pay attention to balloon payments where you have to pay the loan off in full after a few years.