Have a look at Microsoft 365, you should be able to get heavily discounted M365 F3/E3 licenses if the charity meets the requirements, you will be able to manage and secure the devices through Intune, AzureAD P1 and Defender P1.
It will be easy to enroll these 6 devices into Intune, or you can start from fresh and join them to AzureAD then enroll into Intune. You can also configure the apps the charity uses with SSO etc.
File storage, SharePoint / Microsoft Teams will be perfect. You will have 1TB of Tenancy storage, them if you have 6 M365 licenses you should have an extra 60GB of storage. But you can buy extra. Plus each user will get 1TB of OneDrive for Business storage.
If you don't print a lot you can have a look at Universal Print as these will be covered under F3/E3
Nonprofit offers (microsoft.com)