At what point in a new product concept phase should legal documents be created?

I'm an engineer and have a number of new product ideas. I have verbally run some of these ideas past an ex colleague of mine who I used to work with and he thinks there is a market for them and he could raise seed money fairly easily. He is quite experienced on the business side and I completely trust him. My question is at what point in our discussion should partnership documents be created? Should we be drawing up legal partnership documents now, or is it OK to continue brainstorming our ideas? If we should be getting legal documents in place now, then what documents should we be drafting?

Thank you for any advice, Virgil.