Ubercart for subscription

OK, I see. I think Ubercart would work fine for this. Not sure you need a custom module.

Just sell the first annual subscription that includes one (or all three) of the first three reports (have the download files as "features" to the product) and give the user a particular role (e.g., "subscriber") when they complete the purchase. Then, provide the extra reports for sale on an a la carte basis but only available to those that have this role of "subscriber." You might have to use Taxonomy access control or some other access restriction to the product page node but I suspect there's a pretty straightforward way to do it.

Dan Royer