Classified website using wordpress

This project will modify wordpress to be a classifield listing website. I would also prefer to add AJAX for fast interaction. Simple summary: 1. User will be able to sign up. Easy 3 field, username, password and email. Once verified, user can post ad. User can list ad as featured and pay a fee. Featured listing will cost money for xx amount of days. User can manage, add, delete and modify ads. 2. Admin can manage ads, modify and delete ads. Set pricing and choose to expire ads automatically after xx days or keep ads for ever. I have made the screenshot of how some pages will look like. Example the design for homepage, ad listing, category view, full ads view. I will pass this information to successful bidder. Basically i know how to look and feel of the website. The design is in png file and will be given to winning bidder. You will need to create a wordpress theme and/or plugin for wordpress 2.7 and anything else to make this work. I do not want the core files of wordpress to be changed. So, if wordpress upgrades to higher version, this classifield script will still work. So, please make something that can support if wordpress upgrades. I have created a word document with full feature and requirement. Please have a look and place your bids. I think its not that complicated. I will also seek support to upgrade this script in the future. I have a list for future requirement. But for now, please follow the requirement in the word document. I can say this is a long term project. Thanks.