Restarting Insanity workout...

I've never finished Insanity, mostly because it is brutal on my knees, ankles and lower back.
As far as protein powder and/or shakes, it depends. If you are asking about Shakeology or anything through Beachbody, I would skip it..........you can find stuff in stores that's just as good or better for a fraction of the cost.

As far as if it's essential, no, not really. I've had some Insanity workouts where a bottle of water is sufficient, and sometimes (if I'm really burned out) I'll grab a spoonful or two of peanut butter. That usually hits the spot and staves off any cravings for other kinds of junk food.