Messaging plugin needed

Im working on a modeling website. on each models page is a button, “book now”. when a boutique clicks that button it takes you to a page with a form needing to be filled out with some questions on it. When the boutique hit submit, i’d like the message to go to the models email address. to reply i want the model to have to login to the page and respond on the site.

i dont want models to be able to initiate contact with anyone else; just to be able to reply with a message after the boutique requests to book her.

boutiques and models already have login credentials with s2member.

id also like the site admin to be able to monitor messages between the models and boutiques.

any ideas of a plugin or hack that should work