Creating a multi-user mult-blog site

We are interested in building a multi-user multiple blog site. The blog will be similar to another website - [login to view URL], where at the top level there will be multiple blogs, under each blog there will be several categories, and each category will have further selections. The site can also allow creation of blogs for users. We want to use wordpress for this ideally and with custom themes. Please submit ur bids with references ASAP. We are likely to move fast on this project. Bidders with prior experiences building blogs like this will be given a preference. Thanks.