So to your mind, interaction/graphic design is the major differentiating factor, and you're looking at outsourcing that work? That seems roughly analogous to a bunch of business students failing to find a CS major who will do their coding, and wondering whether they should pay someone to develop their idea for them.

I honestly think you're psyching yourself out with respect to the design. But if you really think it's that important, and you really don't think you can do it, why did you pursue this idea in the first place?