Migrate to Multisite?

I’ve been thinking about migrating all my separate blogs (all on the same hosting account) into one WordPress Multisite… Just had a few questions:

1) Is it easy to migrate multiple separate blogs into one multisite network?

2) Can I give my admins individual access to the Dashboard of ONLY their website (like it is with a single WordPress install)?

3) Is everything for each individual domain separate or done together as a whole? E.g., can I activate a plugin on only one website and not all of them (same with theme)?

I figured that I do maintenance on all the blogs and a lot of them have the same plugins installed so this would make it much easier for me to maintain — rather than having to log in to each one individually.

EDIT: Is there anyway to just manage all my websites from one dashboard (by selecting which site I want to manage from some menu in my main dashboard) WITHOUT doing a multisite network?

Thanks!