Freelance Availability Database

I am looking to produce a database service that allows employers in a specific sector to use the web to view the daily availabilities of a range of freelance personnel supplying that sector. The key features are ease and speed. I’d like to use AJAX so that we can offer a very dynamic, graphical and tactile user-interface, with pretty much all functionality and data presented on a single screen. Through the primary front-end an employer would specify a date range and a few key requirements – eg. skills, locality and so on - results of suitable personnel who have free time (plus close matches) appearing immediately. They can then review profiles, exact availability and simply drag prospective candidates names to a scratch pad area to view contact details; offering further functions to print, email, text the list and so on. An alternate front end would allow freelance personal to very quickly and easily update their availability. Yet another would let an agency view the availabilities of just their personnel and update their individual accounts A certain level of complexity will come from the membership system; with four types of user (employer, employee, agency plus admin) plus several levels of admin access for each user type. In total there are at least 8 distinct flavours of user, each who have a slightly different view of the system. Add to this different subscription levels that would cater for different sizes of employer and agency organisation; limiting the number of secondary accounts which can be created. The system will need to incorporate a means to take payment for subscriptions, although the service will initially be offered for free. The design will be clean and simple, in the popular Web 2.0 fashion. I’d expect to provide a guide design, perhaps PSDs for key templates; and wireframes for most pages. I expect you to cut and code all templates and complete designs for the simpler secondary pages. PHP and MySQL is the preferred platform. It may be that an off-the-shelf CMS like Drupal or Joomla (we have experience of the latter) would be useful as a framework for the back-end management of membership, supplementary page content, banners and so on; with the key functionality and interfaces created as custom components. Alternately the project may suite a rapid development framework like Ruby on Rails.