Plugin(s) required to start a wordpress.com-type network?

I am looking to start a wordpress.com-type network and am a little confused...

1) Do i need the Domain-Mapping plugin, or does Pro-Sites have domain mapping built-in?

2) I already have a couple of sites on the network that i set up manually - will these be affected?

3) When i added sites manually, i had to add them to my user account at WPEngine - will i still have to do this (kind of stops it being an automated system)?

Thanks
John.