you would use Data Assist to create the backend pages for inserting product data, see the "How to create an Administrative back-end" and "How to apply Digital File Pro to DataAssist Insert and Update pages" tutorials for details.
for allowing download, see the "How to create an Order History page" and "Creating a Download Center for selling Digital Goods" tutorials.
All of these are available in the how to section of the Data Assist support page:
dataassist/