We have several products that could help you to achieve the desired result you are looking for. The main tools involved in this would be eCart for the commerce functionality, DataAssist for managing records for your products, Security Assist to handle the user login and registration experience, and Digital File Pro for the digital downloading.
These tools are all very powerful and easy to use but would require a basic understanding of using dynamic tools within Dreamweaver. Without this understanding the tools will require some learning to work with and implement properly. I would suggest that you take a look around the site and visit some of the pages for these products and check out the screen shots to get a feeling for how the interfaces look and work. Also if you check out the getting started guides that are available on the support pages they will give you a good idea of the steps you would need to take to make use of the tools.