Restricting site content

[Beejer]

I'm interested in developing an e-commerce website that sells digital products such pdf books, mp3 audiobooks, and avi movie files. I'm going to charge an annual fee for membership, where members can then download the files they want from my website. My site should be fully searchable and organized by categories (such as fiction or non-fiction books, sci-fi or romance books, etc). I need to be able to update the site and add new digital products after the site is completed. I also want a "back-office" portion of the site where I can track membership and accounting, look at site statistics and analysis, and how my visitors are reaching my web page. Does anyone know of any cheap web design companies that would do a good job or any e-commerce templates or software that would streamline the process?