Integrating Automated Invoice, Customer Account and Shopping Cart

Hi,

I’ve seen wordpress used as an excellent client-end interface, where clients can log in and find all relevant information regarding their accounts. Essentially, I need to integrate an automated invoice system (which features support for recurrent automated billing) with user-accounts shared by a shopping cart (e-commerce) system. The shopping cart must also support downloadable products, with each account holder’s products displayed in their user-account pages.

I know this is a big project, but I’m just trying to assess where I should start here. Should I begin by selecting a few different plugins and start customizing them? Will this require changes to the codex?

I would really appreciate any help from anyone regarding how I could go about doing this.

Thank you in advance!
-A