When I first read your post I thought it was a prank, but I was wrong. The reason I was confused is because you are not talking about an NDA, probably disclosure is your least worry, but about a contract.

I was in situations where I considered a contract too, and doing it could be a very good idea, not as much for reasons of enforcement against each other but for making the rules clear, and protecting them against the rest of the world.

It should not be difficult to find a good contract lawyer, from a legal point of view the BDSM part is not relevant.