Programming an add-on feature to eCommerce Platform based on Opencart Version 1.5.5.1

Based on our current e-shop: www.NavitasOnline.com
I need to have "pre-order" feature to add on, which is a module we current do not have in our Opencart (ver.1.5.5.1) admin panel.

The requests of this add-on program are as followings:

1) for a customer to be able to select a larger quantity of a product than is available (especially when some items are with 0 stock quantity but available for pre-order)

2) for the customer to be able to go through to checkout and purchase (process full payment) the quantity that he/she desires

3) for the customer to then receive an email telling him/her that they will receive another email, once the additional products are available, as well as shipping information etc. (these emails designs are also included in this project and would be automatically sent out by the website also easily customized for administrators from admin panel of Opencart)

The budget authorized by my company is £500 (inclusive of 20% VAT) but of course who proposes cheapest price will win the project. No downpayment but after complete testing on the program added-on, we will process payment for 80% on shot and leave 20% to be paid at the end of 3 months warranty period.

Any further questions please feel free to contact me directly, thank you!
This project is scheduled to award by this Friday 21/06/2013, so grab your chance promptly.