Seller financing, how to avoid early payoff?

[Robin McMullen]

My mother is selling a mobile home on a lot to her neighbor and is offering seller financing (California). The agreed upon purchase price of the property is $150,000 with $10,000 down. She would like to have a monthly payment for 15 years and is concerned about the possibility of an early payoff. What are some ways to structure the contract to dissuade early payoff? Also, they haven't agreed on interest rate/monthly payment amount, does anyone have an opinion on a property like this or is there somewhere I can look to determine seller financing rates in a particular area? Thank you in advance for any advice.