Perhaps having a functional prototype isn't the best next step to getting to your ultimate goal. Start by defining as clear as possible the task to solve and then determine the easiest and quickest way to validate your solution. If the 'pain' for your prospective customers is big enough that they're willing to pay for a solution, they're willing to accept limitations with a product in development that solves their core problem.
As a freelance developer for various start-ups I've seen this mistake being made a lot of times: jumping from basic idea validation to product development. Unless you can get a prototype built for free, development is an expense you should put off as long as possible. This goes for situations where time is less valuable than money, which in your case it seems to be.