I figured I would jump in here and help M out with answers.
I got the files here... https://www.cgtrader.com/3d-print-models/hobby-diy/hand-tools/helmet-seven-sisters-star-wars
It looks like they have some open faces after the file was split for printing. I ran the STL's through cloud.netfabb.com and it seems to have repaired all of them except one. I ended up taking the matching side to the corrupt file and using Simplyfi3D to just mirror the mesh and it printed just fine.
I print all my props and accessories in ABS at a .2mm layer height. The ABS is less susceptible to heat warp age in high temp environments and it sands and finishes better than PLA (IMHO)
The helmet itself is now complete and I am currently printing the mask section. I will get M to post up a new photo here shortly on the progress.