I've never done a pep but it's probably the fastest and easiest way to ensure you've got the right proportions and base forms.
That said, when I did a scratchbuilt Samus Aran helmet, long before I ever heard of the RPF, or pepakura, I did the following, junkbuild style:
- take a mannequin head and build on it with literally anything that you can or want to that will yield the desired basic shape(s) of your helmet. I was using foam, tape, tupperware, all kinds of stuff.
- wrap the whole thing in packing tape, clingwrap, whatever will create a barrier surface itself but conform to all the surfaces you created
- add 3-4 layers of wet plaster bandages (available at craft stores) all around and let dry and harden
**at some point, maybe after just plaster if it's sturdy enough - take the built-up head out of the middle, just so you don't have to break a whole bunch of bondo to do it later
- bondo over the plaster, sand (and bondo, and sand, and bondo, and sand....) until smooth, also sculpting and carving details if and as necessary
- paint
- probably sand and paint a couple more times since you should have sanded way more in the bondo stage and you were tired and lazy and just wanted to finish the GD thing.
Now, that said, Master Chief's helmet looks a lot like a motocross helmet, so you may be able to start with one of those and just change the details.