whmcs integration or provisioning

Hi guys i have a couple of questions regarding using pro sites and whmcs for sale of domain, ssl etc.

I use pro sites plugin to manage the levels etc. in the network.
But I would also like to use whmcs for sale of domain name etc, since i can add more product this way.

So do i need both plugins (integration and provisioning) or is one enough?
And is it best to install this on a different domain, if i chose a different domain will the users then have to register with new username etc. (how can i see which purchased domain name is for the right multisite subfolder site) or is there a way to autopush the user and sitename to the new domain name where the plugin(s) are installed?

Hope it gave some meaning