So the use case here is a user wants to get an email notification for a Collection they've created. If any pages in the collection are updated, they want a notification.
I'd like to have it so that the user can just check a box in the Collection criteria interface.
First off, are there any existing add-ons that do this or similar?
If not, can anyone tell me what other add-ons would be necessary in order to achieve this functionality (given that I'd have to write an add-on on top of whatever I'd be installing)?
I am not aware of such a plugin. You may write a script that uses the query() method of a collection for getting hold of all documents found by the collection and check for new or modified content based on created or modified.
For a more general approach, you would query() a collection regularly through a script and store its state (a list of object UIDs) and compare a former (stored) state against the current state. By doing this, you can get hold of new and removed objects.
What you ask for is built in in Plone for Folders/Plone Site.
What is the condition for your Collection ?
Maybe you can use the same (Content type or maybe a TALES expression)?
Add a content rule for object changes and then add a TALES expression as condition (which can call a view or a script to perform the real check) to check if the object is in the collection.