Headwear: A helmet similar to the one featured in http://images3.wikia.nocookie.net/__cb20090326050252/half-life/en/images/c/c2/Shephard2.jpg image, same type of camouflage (titled HECU Helmet).
Gas Mask: A full-facial dark gray gas mask with green tinted "eyes", similar to the one featured in http://images2.wikia.nocookie.net/__cb20090628222431/half-life/en/images/7/74/Adrian_Shephard.png image (titled HECU Gas Mask). I also have a link to a mod that adds certain headwear to to Fallout 3 that you may (or may not) be able to base your model off of, which can be found http://www.fallout3nexus.com/downloads/file.php?id=4698.
Armor: Sleeveless powered combat vest with arctic camouflaged shirt underneath, arctic camouflaged military styled pants, military styled laced black boots (titled HECU Combat Armor). A Combine OverWiki page for the powered combat vest can be found http://half-life.wikia.com/wiki/Powered_Combat_Vest, and (if needed), a full body shot of Adrian can be found http://images3.wikia.nocookie.net/__cb20090517174150/half-life/en/images/1/1d/HECU_shephard_model.jpg.
Again, thanks to whoever is up to the task for this. I think it'd be a good idea if you're a modder that has some free time on his hands. If you have any other questions about it feel free to send me a message or simply post in the thread. I'll be posting some extra links below to provide some more information.
Extra Links:
http://half-life.wikia.com/wiki/Adrian_Shephard
http://half-life.wikia.com/wiki/MP5