I am new to the forums (not really, been reading the site for years now before the name change). I am an experience airsoft player and have been with the activity to for years, and have done it to the most extreme. Additionally I am taking on the challenge of making something as such, but will be doing a lot of testing (spending a lot of money) with materials to produce something as you are asking.
First I would like to say this. The helmet WILL NOT be used to protect your eyes, no matter how you go about it, using a motorcycle faceplate or doing a custom mold with CAB plastic, it simply will not be rated for the use of airsoft. As such you will be require to wear a SEPARATE set of eye protection underneath the helmet (I recommend Bullet Ant). Although it should provide minor face protection (namely the teeth), yet this is not guarantee.
Additionally here are several thoughts I think people need to consider, most are safety related.
-Durability-the strength needs to be strong and flexible. I have to played around with several ideas with fiberglass/bondo/resin with a cover of automotive undercoat to absorb the impact. Yet seeing that I planned on doing a lot of testing, I thought the stuff would surely get destroyed in damage. There fore I am going to invest the extra money and make molds and use Smooth-Cast 320 (which by sounds of it on the Smooth-on website is slighly more durable then 300).
-Padding-Most injuries in airsoft are caused from running around, falling down, running into things. There for proper padding is extremely important. My plans, is to use sports padding out there like shin pads and knee pads and add them under the ODST armor. For the body plates, I am using a MOLLE vest, and the plates will have MOLLE attachments (I have made MOLLE Vests and Pouches in the past).
-Ventilation- Probably something that is overlooked, but is extremely important. You are going to have to sacrifice accuracy of the model to make is more practical by adding vents so fresh air will be able to get in there. This will also prevent fogging.
These are just some of my thoughts. On another note I am expecting and hoping the project will yield a cost of $1,300, yet things will always cost more then one thinks. It is also to note that I have a solid experience on working on art projects of this complicity, and work better with challenges like this.