I think the wrists and butt are going to get the more abuse at first...not an essential piece of equipment in my book when you begin...I don't think you can seriously hurt your head until you start taking jumps...but if you can afford it go for it!
Lliam Neeson's wife died while suffering a head injury on a bunny slope while learning to ski.
All it takes is an edge catch at a wrong angle to smack your head into the ground. One of the worst spills of my experience came from when I was learning how to do toe side stops. I planted my heel edge too quickly thinking I was fully stopped and it sent me flying backwards. I knocked the back of my head pretty good. Thankfully it wasn't during really shitty icey conditions. This is on the hills of Michigan. I can only imagine what would have happened if I was on a bigger mountain on a different trail.
Those types of simple accidents are a major downer knowing that these people who have suffered brain injuries or died could have been protected by a helmet.
When it comes to jumping, the helmet will only do so much depending on your speed and type of bad landing. You can still suffer major head injuries while wearing a helmet if you land a jump incorrectly. However, wearing a helmet while learning will almost entirely eliminate any serious head injury unless it is suffered on the face. Even then, the top of the helmet will take some of the impact off the face.
Of course, there is nothing you can do about neck injuries. Stiff necks suck, but they beat concussions any day of the week.
Wrist guards are extremely helpful. Just make sure you get a good pair. Preferably ones that are a little more flexible so it doesn't break your forearms.
Lastly... Buttpads... They are going to save you from a lot of tailbone soreness from learning. I wish I invested in a pair while I was learning.