I've not tried Aqua-Resin but I have used Jesmonite. It's a polystone resin that you can use with fibreglass, it'd pretty hard wearing if you don't use fibreglass matting/tissue.
You don't need to be in a well ventilated area to use it either. (I was on the fourth floor so needed to use it...
A Nerf Stryfe with an extra grip and stock looks great as an SMG when sprayed black.
Army surplus stores are good for the berets and as Satchmo says, Google the T and any images you need.
I look forward to seeing your progress. :thumbsup