Magento - admin-client messaging system

I am working on a Magento (Community Edition) shopping site that sells kittens and kitten related products. I need an option to send messages between customer and admin user. Something like Facebook Messages or eBay My Messages. It should preferably display messages like a conversation that happened back and forth between the customer and admin. Please let me know how long will it take for you to do the above and how much you will charge for it. Also, while replying, please include whether you will be using an existing extension to do the feature or custom develop one. I have no issues about using existing extensions, but make sure you will be able to configure it correctly.