Questions about: diy-vs-pro

These steps assume you are working with dry wall. You'll do the same thing but working with plaster will be slightly more work. I would suggest using a hole saw to make the hole in the plaster
It all depends on how much you are willing to learn. If you are confident in your ability to do it safely, go for it. If not, do more research until you are or hire an electrician
Only re-grade if you're getting water intrusion or even a damp musty basement condition. If neither of those yet, then don't do anything. Otherwise, re-grading is a good deal of work, but quite simple in procedure with just a shovel