Paypal payments pro integration with drupal 6
For a quick win on this on Drupal 6 I would use Ubercart, it ships with paypal pro integration. And http://drupal.org/project/scheduler

It uses conditional actions to assign roles, give access to file downloads etc. A simple, if perhaps slightly inelegant way would be to pass the node ID of the unpublished content to a product as an free value attribute, then upon successful purchase use conditional actions to publish it, setting an expiry date time on the object properties scheduler uses before saving the node. You should not give users permissions to use scheduler, and I'm assuming you have cron up and running.

Sounds bodgy, but you could get it up and running very fast like that. Check if you like it and still have time to look at other solutions if not.

Commerce module might also be good but haven't used it on d6