I have been printing all sorts of materials on a simple Ender 3 plus a Bowden printer and have no problem printing PolyProp so no problem. for your helmets' the larger printer might be better. I don't think the Bowden feed will be a problem but the choice of filament brad might be. Having spent much of the last decade working with polymer chemistry I can say there are more formulations of Polyprop than you can shake a stick at. The brand I use is OOznest polyP which works for me!! You could also try a semirigid Nylon I use Spectrum PA6 Low warp printing on to a Tufnol sheet. but this is a premium nylon and consequently expensive. Have tried some of the economy nylons but with no luck.

To print a large hollow shape like Helmet it would be good idea to print it in sections to avoid huge overhangs.