The "Multisite" Feature

I am interested in using the WPMU feature of WordPress but had a couple of questions first…

1. Is there any way to charge users monthly to run a subdomain blog? For example, they pay $4/month via PayPal and if they stop paying then their blog is automatically removed?

2. Can you setup a default theme for all new blogs to be using instead of the WordPress default? Also, can you choose what themes are available to them?

3. Where can I find themes that are specifically for people running WPMU hosting companies?

Thank you for your time…