Email a user when they are assigned as case manager?
For sending a message to new case managers - I would suggest using Scheduled Reminders.  


Create a new group "Case Managers". Add all case managers to the group.
Set a Scheduled Reminder on "Activity" of type "Open Case". 
Limit recipients to members of the group "Case Managers".


With regard to the tokens - there has been recent work to support case tokens on activities.  See CRM-19767 and the accompanying casetokens extension.  Note that I'm not sure that this will extend the functionality to Scheduled Reminders - but if it doesn't, I imagine it will get a coder 80% of the way there.