The vanilla Common Data Services already contain a set of default entities that look very similar to some of the entities available in the Dynamics 365 ecosystem. Account, Contacts, Case, Lead, and Opportunity are familiar ones that we can find in the Dynamics 365 Sales module.
As a rule of thumb, if you require entities that look similar to the default ones, reuse the entities rather than creating new ones from scratch.
Strong security is one of the main selling points of the CDS. You can secure your data model by navigating to Settings | Admin Center (also accessible from https://admin.powerapps.com), selecting an Environment, and creating User roles and Permission sets, as shown:
For more information about security, read the following article at https://docs.microsoft.com/en-us/common-data-service/entity-reference/security-model.