To further build on Ryan's ideas, I'll discuss the downloads aspect. The custom install script should also be able to create a special folder outside the web root for each artist site that's created. This directory will be set as the place to store artist's files that they upload to the site. The hook hook_file_action will allow to realize the preview files and thumbnail aspect of the site. You can create a custom module that will act upon new files that are uploaded. In your case, you can have a custom module that can open video or audio files, trim it down to a small preview, and then create a copy in the public files directory. You can do something similar for thumbnail images as well.