In my 5+ years of developing for these types of people, they need far more than someone who will "build your prototype." Generally they need guidance in knowing what sorts of decisions need to made, and then in making those decisions.
I'm all for string to provide a turnkey prototype development service, however I would fear most ideas and wireframes will be doomed from the start without push back for a well thought through MVP, which is likely not part of this service. That could lead to disappointment, over-reaching, and the unreasonable expectation of multiple iterations.