Job Club

I'm working on a website for someone who wants to set up a career club. The site will be based on Wordpress and incorporate taking payments for services through Paypal. The sequence would be:

customer buys one of two services through Paypal (basic and premium only)
site owner gets notification of purchase
customer gets email saying welcome + username and password
customer can then login and change password if he or she wishes
customer then has to choose a coach from a limited selection and also option to upgrade from basic service to premium if they wish
this choice is recorded and sent to the coach and the site owner
once logged in, other site content may be available

Can you tell me if you would be interested in doing this part of the site development and give me a rough idea of how much it would cost? The idea is that I would work up the general structure of the site during December (with input from you), the finishing touches would be made in January and heading for a launch in late January.