The biggest challenge you're going to face is that the state of the art in NLP is not sufficient for chatbots to conduct a prolonged conversation that is either

The biggest challenge you're going to face is that the state of the art in NLP is not sufficient for chatbots to conduct a prolonged conversation that is either useful or interesting. This will lead to disappointment on the part of your users, who have probably been exposed to the hype or expected a human. The more modest approach of mapping a Web UX to a sequential chat leaves users at best indifferent, and at worst annoyed.