Project starting point advice

Hi
I am developing a site that is similar to oDesk or peopleperhour etc - I need some advice / suggestions on plugins and direction to go.

The things I want to achieve are - to have a membership type site that jobs can be posted on for free.

Also 'workers' can put their profiles on and reply to and posted job.

Also possibly the bit I don't know how to do yet, I would like each client to make payments to the workers but a % is taken as an admin fee?

Help much appreciated.
Regards
Mark