After explaining your business and idea, ask;
What do you think about the potential and viability of my website idea?
What opportunities and ideas can you suggest?
The key to this question is you don’t want to hire a web designer that doesn’t believe in your business or idea.
Have a look at their website design portfolio. Is it quality work? Is it in the style that you like?
Make sure that some of the sites that they show you are of a similar price point to what you are budgeting. You may contact a few of their clients to see how they found the design firm to work with