New App creation, using existing solution

Hi,

I'm creating a new D365 app.

I'm thinking of using an existing solution and modifying it.

Once modified to fit the brief. How will I then only allow certain users to have access to this new application.

Would I need to create a new 'contact' entity. I'm hoping this will be populated by an API from another application and can accept manually entered data.

or will just applying Roles / permission do the trick.

This new app will also need to incorporate Marketing.  Will probably use this as the initial solution used.

I think there is a spare D365 Marketing Application??

Any advice / ideas as to how to create a new app would be grateful 

   
Regards

Chris