Seamless, automatic multisite domain purchasing and mapping

I want my users to have a seamless, branded experience creating their websites via my multisite install.

What I want:

1. They arrive at my primary domain (
2. They register a new site (
3. They purchase their domain name ( via without seeing any other domain registrar info--they only ever see their domain name, nothing else.
3. Their new domain ( automatically maps to their subdomain (
4. They use, view, and edit their site via and

Currently, I have a new multisite install on via, which includes a dedicated IP. If I need to switch hosts, I'm up for that. But I really like managed WP hosting for its security and backups.

How do I make custom domains seamless and idiot-proof for my users? I don't want them fussing with ANY techy bits. Better if they never see them at all.