Template for Setting Up A User Login and Tracking System?

I’m working with a client’s existing Flash website.
She would like to set up a membership area which would allow users to sign up to:
access special content,
share data with other registered users,
and receive mass e-mailings.

In addition she wants to set up a basic web store which would allow anyone to purchase merchandise ala Pay Pal.

The website is hosted in an IIS / ASP.NET environment.

**Can anyone recommend an existing prototype package either commercial or open source that I could use as a starting point to develop this site?
I’m pretty comfortable with doing forms in HTML or Flash.
The server side and database where I particularly need some advise.

Any guidance would be most welcome.