Ye' old general discussion board. Basically, for everything that isn't covered elsewhere. Come here to shoot the breeze, shoot your mouth off, or whatever suits your fancy. This forum is not for asking programming related questions.
Sounds like what you need is a site script like at sitescripts.com or something. However, it takes experimentation. Another avenue is outsourcing it, such as putting the digital content on some other site and then they take a small percentage if the sale goes through them.
For me, I recently had a client who had a very small budget and he wanted a lot in that budget. He asked me for a CMS system, but I didn't like much of what I saw out there, so I wrote my own with categories, subcategories, and articles. But then when he wanted a product catalog, I simply forked the CMS code, added in prices on the articles, created a button to click to get the price translated into other currencies (thank you Google), and then added the PayPal Add To Cart and Checkout Now buttons, which were painfully easy to integrate now that PayPal has fixed their sandbox system and has made their API more reliable.
Also, recently I had to build a way to implement a subscription for site credits, and I integrated with Protx.com because that's what the client wanted. Well, it was a little more confusing in the docs compared to PayPal, but ultimately it worked somewhat like PayPal once I studied it.
And then the other avenue is to request someone build it for you by placing an ad on SitePoint or eLance or something.
Magento supposadly has downloadable product support. So does my software ( view @ link in signature ), but mines not free, or open source... so Magento wins there..