Hi,
Dynamics 365 is not required for CDS or Model-driven apps.
If your client is migrating to Office 365, PowerApps and Flow are included in that O365 subscription at no additional cost. Likewise, if your client has a Dynamics 365 subscription, PowerApps and Flow are included in that. It is just one more way to get access included.
If your client is interested in using CDS or building model-driven apps, they would require a P1 or P2 subscription. Here's a blog post with more information on which plan is right for your needs:
https://powerapps.microsoft.com/en-us/blog/business-model-may/
Let me know if that helps.
Brian
What you’ll probably want to do is add the data to a Collection using the Collect function in the OnSelect property of a button.
There is also the LoadData and SaveData functions.
https://powerapps.microsoft.com/en-us/blog/build-offline-apps-with-new-powerapps-capabilities/
You could do this, I feel like this is the wrong solution though. There are a number of solutions out there already for this pre-made. https://zapier.com/blog/best-meeting-scheduler-apps/ However PowerApps does have the ability to interact with Office365 profiles so you could read/write a list of calendars.