Here's a way to do this using Paypal buttons rather than paypal enrollment:
With a paypal button you can set a specific landing page after successful purchase, make that an orphaned page which has to be viewed to be complete. Make all the "premium" topics requiring payment release based on viewing that page only. https://www.paypal.com/cgi-bin/webscr?cmd=p/pdn/tech-overview-singleitem-outside
That should give you the option to allow students to see the full course but only if they make a purchase (and you don't even need to use Paypal's enrollment plugin!)