First off, I love this idea and don't listen to the naysayers.

Second, I would strongly recommend getting a lawyer to draft up some sort of agreement that specifies what happens if the project isn't up to spec, etc. It would essentially be a basic contract. I know there's always the temptation to say "well the legal costs would be too much to enforce it" but that's not always the case. This is garden variety stuff here. You don't need to protect every scenario to provide some value to yourself.