I'll do my best to respond to that.
-Yes a lot of brushes(10 to start). The outside coat you will probably only need 1-2 brushes per layer.
0- Yep sounds good, I didn't use a lab coat but thats smart thinking, hope you have lots of disposable gloves.
1-Smart, I bought a small 2oz measuring cup from walmart to measure out my resin before pouring and mixing in a red solo cup.
2-Yes per oz its I believe 11-12 drops of hardner.
3-5: tip of advice is maybe even buy some dollar store or walmart painting brushes for the very small areas like the ears. Also once you have a layer down and on it you will want to watch and smooth it over because it will "drip" or move down the surface and don't want any globs you have to clean up later like I did.
After you are done with just the resin coats you will need to either fiberglass cloth or matt the inside. that is where you mix the resin and coat small sheets of fiberglass cloth or matt and line the inside. Check the tutorial index. Once you fiberglass cloth/matt the inside (2 layers) you can rondo the inside of the helmet if you so choose(look at the tutorials again). I am waiting to sand until after I have fiberglass clothed and rondod the inside of my helmet (hopefully this friday...) because after you fiberglass cloth/matt the inside it is extremely durable and a hefty product and will withstand sanding without budging.
I never filled the inside because I will just bondo after the resin/rondo it, which is the step you want to do after sanding down the resin. Bondo the outside of the helmet and sand it down to a smooth surface.
xXDashIVXx would give you the advice on going very light coverage with the bondo or you will be sanding for ages.
Anything I missed others can chip in.