I actually used a Razorknife with the Hook edged blade to cut out the trash can plastic. Worked really well and was very fast. Just needs a steady hand so you don't cut yourself =P
True, but a scratch build will come out looking better (if you put in the time and effort). If you are new to working with tools and fiberglass &c., go with a rubies (OR, wait for saz to finish his helmets and get one of them...great looking work there!!!)
Either way you go, if you have any questions on what to do, just ask and I'm sure SOMEONE here can answer any question you have!
just to make it hard for you i would go heavy mando. heavy is good, a sign of reliabillity. if your armor doesnt work you can always through it you enemy