Should you charge before building an enterprise product with a client?

Have an idea for a claims management application. I got one company who is in the selection process amongst other such software but they want to work with me to build something from ground up. I want to build a claims management software but have no inner insight into how they operate and having a client let's me define specifications that is reflected by real world, and avoid having to build something nobody wants.

Now this seems like it might take 4 months, however, to work 6 hours a day for the next 4 month seems quite impossible. Then again, I would be able to build a product which can be sold to other companies.

Should I have an intention to buy contract ready or should I charge an hourly rate? I'm afraid that this might scare them away but then again I can't work for free so what should I charge?

The other software consultants in this area seems to charge $200 / hour and the software seems to range from $20,000~2,000,000 depending on the end user.