how to develop a network of sites like edublog or wordpress.com

I want to create a specialised network of sites like Edublog. I intend to use Prosites + whmcs provisioning plugin. But need the house's advice on the following:
1. What infrastructure is recommended for a start?
2. What is the standard infrastructure requirement for at least 100 sites if they also have Woocommerce installed? At what point should I upgrade to a typical infrastructure for this network?
3. Any recommendations on how to create a staging site for my customers?
4. How can I ensure successful updates of all plugins and Wordpress version even when thousands of customers are on board i.e. ensuring nothing breaks?
5. How can one test (quickly and easily) that nothing got broken on the sites after the upgrades?
6. Is the WHMCS Provisioning plugin compatible with the latest version of WHMCS - WHMCS 6.0?

Many thanks for anticipated responses :slight_smile: