I've implemented such a feature, after the order invoice, a list of products purchased along with download links are generated like so:
1. producttitle: (download button)
2. producttitle2: (download button)
Clicking the product title will take them to a the details of the product (same link in the email), download button will send the user the file.
If anyones interested PM me for details, there are only two files you need to edit for this functionality.
