business arrangement as a private money lender

I am a realtor who is teaming up with an experienced General Contractor on my first flip. My role is essentially a private money lender for half the down payment. What type of agreement/contract should I have with my General Contractor? I am hoping we don't have to form an LLC as I'm not sure this arrangement is going to work for the long-term. My general contractor has an LLC for his construction business & I have a LLC for my real estate business. Do we buy the property under his LLC? Is this something our hard money lender can help us with? I honestly don't know what I don't know & need guidance.