Design & Installation of Private Messaging System Like Facebooks

Hello! I'm after the design, build and instillation of private messaging for an already established community website. The system will need to integrate into websites infrastructure, be spam proof (users stored in sql database, and are either trusted or not trusted.... not trusted members will be required to complete a captcha, trusted members will not). Message counter / notification needs to auto update every 1 min without page refresh. Messages will be submitted via ajax like Facebook to create a live "conversation view". Messages will be sent via the members profile and clicking send private message, or by going to new message page which will have an ajax drop down / auto suggest with profile picture when users starts typing in a members username.