Crowdsourcing Website for MS Office Help

I would like to build a website, much like freelancer.com that allows people to submit projects for help with Microsoft Office products like Excel, Access, Powerpoint, Visio etc. I'd like to charge users for posting projects and have a forum in which other users can bid doing the work. I'd also need a rating mechanism for both buyers and sellers. I'd also need the site to handle the transactions of money, milestone payments, releasing of funds etc.