When posting an answer, the question author's receives a notification. Let's assume I delete my own answer, edit it and then undelete it. Will the question author receive a second notification?
Only the creation of the answer notifies the question author. However, you can leave a comment on their question letting them know that you updated your answer.
No, the OP won't be notified about of any changes of existing answers.
You can notify them yourself with a comment at the question though.