Can I resell hosting using software such as WHMCS?
If the tool allows you to link with Plesk, then yes. Usually, these programs include the possibility to communicate with Plesk, check its documentation carefully.
Can I provide my customers with an SSL/TLS certificate for free?
For every website you create, you can generate a Let's Encrypt SSL/TLS certificate for free.
Can we use these services for personal use?
If you need to host several web applications and want to fully customize their settings (within the limits imposed), you can use these offers.
Can I upgrade my service in the future?
Yes, if you need more options, you can upgrade your service at any time from your customer area, with no loss of data and at no extra cost.