Your web site is your face to the world. If you settle for amateurish work, you tell your potential clients not to expect professionalism from you. If your site is garish or tacky, that impression will be transferred to you.
Just because you're not a big corporation with a team of designers working for you doesn't mean you have to settle for a second-rate site. You can still afford to employ professional design help in order to be seen in your best light. In fact, you can't afford not to.