Membership Help, Plugin, Idea.. something

Alright, I know this topic has been discussed in a few threads and I have read a few of them but I still don't think it has been addressed based on its need.

We currently have a multi-site blog powered by MU and we have supplemental content (downloads, images, files) etc that we only want to offer up to individuals who are "paid members," say they pay $10/year and they get access to all the supplemental segments and files.

We don't need to restrict pages, posts entirely, etc just a "block of a post" say a file that we would insert via a custom field, something.

If they user isn't logged in it would say something we could customize say "this article has supplemental content, click here to login. Not a mamber, signup today!" however if they are logged in and paid up, let them download. Paypal Payflow/Payments seems to be a popular merchant gateway for this.

Ideas, suggestions, adaptations?