If you use PayPal's IPN or their Pay Flow Pro stuff, you can capture their billing information and that's about it. You can't store credit card information without meeting some strict standards about ensuring that the data is secure. That may very-well be over your head.
Anything you want to store about the people downloading your files, you'd have to have a form to capture whatever you want.
If you want to capture the IP of the user when they download the file, you have to actually send the user to a php file which writes to a database or file, and then passes the file through itself to the user.