Final answer:
The most suitable action is to migrate the finance department's site collection to SharePoint Online, then implement Azure Information Protection and Azure Rights Management for data protection and management. OneDrive for Business is not recommended for departmental use as it is better for individual storage solutions.
Step-by-step explanation:
The task at hand is to migrate existing services from an on-premises datacenter to Microsoft 365. Given that the finance department's site collection is encrypted with third-party software, the best approach would be to first migrate the finance department site collection to SharePoint Online. SharePoint Online is a Microsoft 365 service that supports encryption and can seamlessly integrate with Azure services for enhanced security.
After migration, implementing Azure Information Protection would be a wise step. Azure Information Protection allows for the classification and protection of documents and emails by applying labels. It can also track and control how the information is used.
In addition to Azure Information Protection, utilizing Azure Rights Management (sometimes part of the broader Azure Information Protection suite) will help to protect sensitive information. This management service uses encryption, identity, and authorization policies to safeguard files and emails across multiple devices.
Moving the site collection to OneDrive for Business is not ideal for a departmental solution, as OneDrive is more suited for individual storage rather than an entire department's document management and collaboration needs.