Hiring a professional to help you with the design of your website is a great idea. This is particularly true if you do not know anything about design, what looks good on the web, etc. Of course, there are tens of thousands of web design professionals out there. For some, this makes the selection process more difficult. For others, it simplifies things because they have more options.
Above all else, you need to get samples from each and every web design firm that you are considering. What have they done in the past? Do you like the look and feel? Do you have confidence that they could duplicate the same thing for you? When you see samples you will feel much better about making a final decision.
It would be in your best interest to ask for samples that are related to your particular project. For instance, if you need a sales letter template you should ask for similar samples. This way you can compare apples and apples instead of trying to piece things together and hope that the project turns out the way that you are planning.
When you hire a web design professional you are getting exactly what you want and need. That being said, not every designer is going to be right for your particular project. The best way to cut through those who are not qualified is by requesting samples. Soon enough you will have ideas from many designers, and from there you can make a decision on which one is right for the job!