The proposal should include: The amount to be borrowed (principal). Interest rate (You should offer, even if they're likely to decline. For a long-term repayment, 2% to 4% is reasonable.)
Outline how you will handle disagreements or misunderstandings. Agree to celebrate each other's achievements and special occasions. Specify if this is a temporary agreement or ongoing. Include a clause that allows for changes to the contract as the friendship evolves.