Question for WPMU developers re modifying WPMU

I want to put up a news site (using either an attractive existing news theme-or a newly created news theme) that would allow many people to register and blog for free, but there may be people willing to pay to blog about some subjects. There may be 25 categories that people would pay to blog about. I want a potential blogger to be able to select one of the “pay to blog” categories and have the exclusive right to that category. I want them to be able to have a “lock” on that category, and I want them to pay for it. I’ve seen the “pay to blog” plugin but I don’t want to put people through the PayPal/”credits” hassle; I prefer just to have a page that lets them signup with a debit or credit card, and the card will get charged each month until they decide to cancel?

Can what I’ve described be done in WPMU? If so, does anyone have an idea about the cost and length of time to develop the site?