While it may not be necessary to get money up front, you will need a contract signed. I'd pattern the contract off ones used by architects and the like, specifying who owns the design, who may use it, and placing a value on it.
It's not so much to produce direct income; what you're primarily concerned about is the folks using your work to piece out the work. You don't want to simply give them a shopping list for a trip to Home Depot. You also don't want to lose the job to another, only to be still on the hook for liability.
On another note, I suggest you be very clear, in your mind, as to the way the church operates. For example, a community center I recently did has lots of outdoor receptacles, on several circuits - all because, in the summer, they use it for outdoor weddings, concerts, etc. Since the job was started in the Fall, I would not have known this without having detailed discussions with the people who will use the place.
