Building a static website in Dreamweaver, and building an e-commerce platform are two very different things.
Html is a very easy language. Quick to learn. Current standards of CSS make it a little more complicated, and it's not the easiest thing to master, but it's still easily obtainable for a novice to produce an acceptable product.
For the forms and e-commerce to work however, you need to get into some server-side scripting (something like PHP), and this really separates the men from the boys! You're talking a real programming language. With stuff that will go right over the top of your head and have you pulling your hair out within the first hour. That's before you even get the software installed on your machine!
Now there may well be stuff out there already. There's certainly a lot of e-commerce platforms - finding one with the download option and your exact requirements will be the tricky bit.
If you really wanted to take the DIY approach, it's certainly not impossible. But be warned.
A login is fairly easy. Though you don;t see how you want these to be managed. It can quickly become a lot more complicated.
You can use PayPal buttons to purchase (PayPal will give you the code), and this will cut out a
lot of work involved in building a fully fledged e-commerce system. Show the buttons only when logged in (easily done), then take the customer off to a download link when done.
There will still be some work involved in the above (don't be fooled), and potentially some security issues (in the sense that more than likely, the more knowledgeable will probably be able to get to your download links - if that's something you can live with..).
It's not the most professional looking solution either...but hey, you get what you pay for
