Shopping Cart Modification

Project Description:
We are looking for a coder to work with to modify an existing open source cart that will integrate into our Wordpress site. The actual cart is still to be decided (oscommerce, magento, xcart) but the biggest needs from the coder is:
1. Ability to write very clean code and logically think through ways to break it
2. Lots of experience with carts and must be excellent in PHP , Javascript and possibly AJAX.
3. Must be able to start this week.


Here are cart requirements:
1. Operate on Linux platform instead of Windows so that it integrates well with WordPress CMS
2. Integrates with Authorize.net CIM - We use CIM for subscriptions.
3. Will need a script so that the cart will inject some basic order data into a separate mssql db to integrate with our shipping software.
4. The cart will need to generate the following automatic emails.
a. Order Confirmation
b. Tracking information (Package has been shipped)
c. 5 day notification on subscription recurring shipments upcoming
d. Some type of auto responder platform capability would be great
5. Cart must have robust subscription module with ability to set option of 28 days (1month) and 86 days (3month) automatic orders.
6. Ability to create orders from admin ( phone Orders)
7. Ability to track friend referrals and then credit their account ( A friend buys they referred and a $10 store credit gets added to your account)
8. Will need a custom feature on the shopping cart page itself that lets the customer pick a value between 1-40 ( will be handled through javascript)
9. Implementation into our Wordpress site so wordpress experience is a must.

I have a good example of a cart I can show you but first let me know your experience with carts and thoughts in your bid on how you think it would be best to build the cart.
Thanks for your time and look forward to a great project.