Your friends' suggestion is actually good and a way for you to kick start an eCommerce site less the knowledge. Just be sure to use your selected theme on the intended content management system, which is OpenCart. The only disadvantage of using ready made templates is that your business front-end will appear the same as the other websites whose owners bought the same template you acquired. If you haven't seen it, the template you are eyeing for had been purchased 3304 times, and that just means your website will be 3305th duplicate. Should you hire a professional designer to create a unique front-end for your ecommerce site? Ideally, you should. But, if you're afraid to spend money for the advantage you will get, then use ready made template and let your site be treated as a copy-cat.