Magento extention

I need an extension to integrate with the checkout process in Magento, that records how visitors heard about the site.

This must do the following:
* Be a module – NO CHANGES TO CORE FILES
* Integrate with Magento onepage checkout – Displays in billing info section
* Recored information to the Customer record
* Work for both Guest and Registered customers
* Show on the customer record
* Shoe in the customer grid
* Be editable and updatable – either through the admin interface or a config file
* Cope with sub choices – Example below

Example of functionality

How did you hear about us? (dropdown)
- Search engine
- Friend or family member
- Magazine (please specify) --> Shows textarea
- Previous customer
- Show (please specify) --> Shows dropdown
-- London
-- Birmingham
-- Manchester
Mailshot
Other (please specify) --> Shows textarea

The site is built on Magento CE edition version 1.7.0.2