You should be able to use the built in Email objects to do this.
In the IME, go to the System menu and select Email>Create Email.
Under the When to Send tab select Task Completion and select the desired task to tether the email to.
Create new custom data elements as needed in your email. Most of the task completion information should be available in the Tasks and Events category.
For further details, you can review the documentation here:
https://wiki.ca.com/display/CIM1265/Create+Email+Notification+Policies