Yeah man; that's a good idea.
I'm currently putting together a scout uniform and just finished my stormtrooper about a month ago. If there's anything I've learned about this hobby during my road to joining the 501st it's that shortcuts are never worth it. I spent 11 months working on my stormtrooper and even though I've been approved for trooping I'm still not totally thrilled with it, lol.
Occaisionally you'll find a decent deal on a known commodity in the "for sale" forum section here, but I've learned that the popular sellers KS, MLC, LSFU, SC etc... are popular beacause of their great track record. Sure you might save $100 buying something on the 'Bay, but you have no idea what you're getting, where it's from and how much work you'll need to put into it to make it acceptable. Since that sellers items are fiberglass, it would make it even more difficult to mod and make the necessary changes.
I finally decided to go with a KS helmet and the extra $100 I'm spending to get it is a very worthy investment in peace of mind. You'll have a helmet that's pretty much good to go right out of the box. It will likely just need some sort of interior suspension or padding to make it really wearable.