MarketPress and Membership: How to combine them to let members download a product for free while vis

I want to combine the membership site where I use MarketPress for the product listing and free download for members and an external shop I am using. My idea is the site works similar to the wpmudev one: members can download a product for free while non-members (or from a different member level) can purchase an item. Would this be possible and if how?

What's more: I have to use the link to the product file for security reasons and not have MarketPress distribute the file. MarketPress may handle downloads fine but this is not 100% perfect. I am using my files outside of my webroot, so even the link can be guessed or shared, the file can't be downloaded. So what I actually want to do is: show members this external links instead of the Buy Now button.