Hi Mike,
Instead of using a custom notification email you can create different tasks at the template level for each email reviewer and in build the standard text in task description. You can condition control the visibility of these tasks if you want that certain task to be activated only when a certain field value condition is satisfied. Any portion of the text that cannot be predefined can be added by the project owner before submitting it to the reviewer.
Hope this helps.
Regards
Vilas