All the features you have listed could be done using a combination of our extensions.
I think you should take this project one step at a time, so you might want to finish the user registration and then work on the design submission and then start working on the admin section, and if you have more specific question, we should be able to help you.
I will describe in general what extension you need to use for each of the feature:
1. user registration -> Security Assist
2. Design Submission -> DataAssist, Digital File Pro, and Universal Email
3. In the Back end, you will use DataAssist to create the admin pages. So it will create a result page of all the designs submitted and an update page to make update and then you will use Universal Email to send out emails to users.
4. on the front pages, for the check out, you could use eCart to create your check out pages.
There is a lot of small details that i probably did not cover, but this should help get started.