Question about structuring private money loans

[Simon Everett]

Hello guys! Im getting closer to buying my first property. If I have a partner with private money willing to put up the down payment, how do we structure the deal? I believe the partner would be willing to do something like a good chunk of equity in the deal in return for his money. I would also like to know your opinions on paying interest on his money instead. For example, gives me $20,000 for down payment at 5% interest to be paid back by xxxx date. 

Is this legal with traditional loans? Would I have to season the money in my bank account? How would this work legally? JV or promissory note?