A template is a good place to start if you are on a budget, but in the long run, you are definitely going to want a custom design. It is all about branding - find one major e-commerce retailer that uses a template
You want to make an impression in the 5 seconds you have to grab their attention, and you want them to remember your site as being unique - not looking like 700 other sites out there. A good designer will capture the 'feel' of your company so that it is instantly recognizable as soon as someone logs on to the site.