From the Admin menu click on the Site Settings page. There look for the Security setting in which you can control if registrations are turned on for your website.

Once you have registrations turned on. Create a new page, put a Documents module on the page and upload the product to the documents module. You can change the page or module settings to make the page only visible to users who are logged in, so they can't download without being logged in.

You can also, through the File Manager, make the folder where you post your product for download Secure so that only users who are in a certain role, "Registered Users" can download the file, that way people won't redistribute your download URLs as they won't work unless logged in.

You would need to go into the Language Editor to change the emails. though, within there I am not sure which files offhand control which emails.