Looks like you're wanting to have a feature akin to an Activation Key used in licensing downloaded software...

Currently, there's no feature (or bolt-on module) in Zen Cart that performs that manages this (rather detailed) process.

But why are you wanting to create a separate password???

If you create a downloadable product, when a customer purchases that product, your configurations "point" the customer to the download... and ONLY your customer will have access to that download. (The .htaccess control in the DOWNLOADS folder prevents unauthorised people from getting in there.)