Yes, you will be able to accomplish these goals using WebAssist plugins.
you will need to use eCart, for the online store. DataAssist for the catalog administration pages. SecurityAssist to restrict access to the admin catalog, as well as registration and login of users. Digital File Pro to offer file download.
We have some tutorials that will help you with certain parts of developing this on the eCart support page in the How To Section:
How to apply Register/Login functionality to your checkout page
How to create an Order History page
Creating a Download Center for selling Digital Goods
on the DataAssist support page there is a tutorial for creating the Admin back end to manage you catalog:
How to create an Administrative back-end
You will probably also want to look at the Security Assist recipe for creating user level authentication since you will have shoppers and administrators in your users table: