there is a tutorial here for creating a download center:
http://www.webassist.com/tutorials/How-to-create-a-download-center
to limit to only one download, you will need to add a download count column to the orderdetails table, set the default value to 0.
In the recordset to populate the download page with download links, add an additional where clause to check that the orderdetails.download count is less than 1.
In the download behavior, you can set it to increment the download count column for each download.