I'm a US based developer and I'm interested in working on this project with you. Here's the setup I would propose:
1. You have a series of Paypal buttons on your page that your user can click on to pay. Attached to the buttons is a code indicating the file type they want to purchase.
2. When the purchase is complete and verified by IPN, the transaction will be added to the database. A unique URL is generated for this transaction for the specific file the user purchased and is emailed to the user. Note that this cannot be on the page that the user is referred to after completing the Paypal transaction because the IPN notification process occurs independently of the user returning to your page, and typically takes a few seconds to verify the transaction with the credit card company
3. The user can click the URL to access their file. Every time the user visits this URL, their access is logged, and after a certain number of times, or after a certain period of time, the download URL is no longer valid. The custom URL will be setup such that the true location of the file is never revealed.
Can you tell me more about your existing site and the type of hosting your site is on? I would need FTP access and access to your phpMyAdmin interface to set this up. Also, since I won't be developing any new pages on your site, I want to confirm that no design work is required
Let me know if you have any further questions or are interested in proceeding!