WordPress Website E-Commerce Extension

An already completed wordpress website has to get extended by an e-commerce plugin. The website is suppose to offer digital products (software) that should get provided as download after completing the purchase. The digital products are supposed to get displayed as a list with the product name and a few specific information. There are different series of products and each series has about 40 single products. The cart has to be displayed underneath the lists. As a guideline for the design I will provide an illustrator file with precise measurements. As the product range is growing over time, I am looking for a solution that enables me to easily extend the shop by new products. Magento, WP E-Commerce, Shopp are systems i believe are suitable, but I'm open for other suggestions. Please only bid if you have experience with e-commerce and wordpress projects.