Is Multisite what I need

Hi
I dont know if multisite is what I need for my project. My plan is to build a promotion/selling website on example.com and a demo website on demo.example.com. The product website will be a buddypress based website for groups to collaborate with each other and It will be un-accessible to public (think basecamp but for schools/churches etc. etc.) I was hoping to find a way of install a complete duplicates of the Demo install to other sub-domains as required. It would also be nice to be able to update required plugins across all the sites at the same time.

If multisite is the way forward would I need to set the root install (example.com) as the multisite super admin even though its not going to require all the plugins/theme etc. or can I set demo.example.com as the super admin.

Thanks for taking the time to read this and any opinion will be greatly appreciated as multisite will is completely new to me!!