I'm looking at starting to collect bits and pieces to start my armour build. I have seen replica M1 army helmet on eBay. It says its for adults/kids and is made of PE. The colour is olive drab and comes with DPM camo Ripstock cover. Is this any good to use as the base for my helmet or should I try and use a metal helmet. Will I need to use the camo rip stock cover or will I need to just use the plastic helmet and paint it.

The camo rip-stop ( so-called because the grid pattern sewn into it, well... stops rips ) will be the wrong pattern unless the helmet is being sold as an Aliens helmet.
You're best, if you're not getting an actual Colonial Marine helmet, to paint it up yourself at the same time as you do your armour.

You'll also need to fashion a front strike-plate for it, if you want total accuracy ( Although it does look OK without. Think the main reason for it being there is so there's room for the drop-down IR sight they used in the film, but if your head is small enough, you might be able to get away with only cutting the helmet liner... )

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MSgt, UKCM 42nd Reg., 1st Batt. : Hadrian's Wall. UKCM

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And if you use a helmet cover on it, that would hide the lack of front plate a bit anyway.

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Steve
Corporal in 69th Regt. 1st Batt. Homer’s Heroes.

"I'd like to die at the age of 80 in my own bed, with a belly full of wine and a girl's mouth around my c**k."