Domain mapping plugin with WHMCS provisioning

Hello, I have managed to get the basics of WHMCS MU Provisioning and WPMU to work with subdomains. However, now I had some questions in how to set up acutal domain names.

1) If I use the domain mapping plugin, what is the reoccuring time/effort involved? As an example…A customer arrives on my site and decideds to make a purchase. He registers a domain name on my site because I have a Enom account for WHMCS, and the multisite is created. At this point do I need to manually associate the domain my customer registered or will the domain mapping plugin handle that for me? If it does not handle it automatically, what do I need to do to ensure they have a fully functioning website on their own domain pointed to a multisite install?

2) How can I test the integration of domain mapping with whmcs mu without registering a bunch of domain names?

3) I currently have WHMCS for my reseller plan. I have the admin account set to have 2GB out of the total 50 GB provided to resellers. Now with WHMCS MU Provisioning, my assumption is that the sites are created under the 2GB limit and not the 50GB limit. Is this a correct assumption? So if I got enough customers that they exceed 2GB in storage, I would need to increase the space for my plan to continue?

Thank you for answering my questions, I know its long, but I would like to make sure I’ve educated myself on the whole process before soliciting customers.