Yes, TextAloud is available as part of the Volume Purchase Programme (VPP). You can read more about how you can access VPP here: https://support.apple.com/en-gb/HT203043

Currently only the voices provided by Apple are available offline. You must have an internet connection to access the Microsoft voices. The voice selector in the app will show you which voices are available online and offline.

Once you have downloaded the free version of the app you can go to Settings > Voice Usage Stats. This will show you how much time you have left and will also allow you to purchase unlimited time. You can purchase unlimited time via the an in-app purchase from the free version. You can also download the full version directly from the app store here: https://apps.apple.com/gb/app/textaloud-pro-text-to-speech/id6446679219