It all depends on your interests and what YOU want to do with your collection. Just like weapons (old jeeps too), all of these things are only original once. You have probably read the discussions on Garands - do you want to restore a mix-master to all original parts (it still won't be "original") as a show piece, or do you want a reliable functioning rifle you can take to the range and enjoy shooting and/or sharing with others to tell the history?
The helmet shown as my avatar is a WW2 front seam swivel bail shell that was a real train wreck of repaints, past paint removal attempts, (even a large area where looked like someone tried to burn off the paint). There was no texture left on the outside (cork or sand) and no chin straps. I wanted a decent looking representative helmet for living history so decided "What the heck - I can't hurt this one!" and began to restore it. I started stripping layers of paint and uncovered a name and service number painted in white on the inside. I went ahead and completed a restoration with new paint and texturing, but protected the name and number I had found on the shell. I guess my point is, even on the worst of basket case shells, there may still be a story to uncover.
I restored a basket case WW2 helmet liner (someone had converted it to a skate boarding helmet or something) with repro webbing and leather chin strap, added a repro sewn-on chin strap and helmet cover to the shell, and put the two together. It makes a nice set for living history, which I am not afraid to use and wear, plus I am able to "weather" the whole ensemble without worry of damaging a true artifact. Warning though, I probably spent $15 on the shell and liner originally, but have over $100 in repro parts to do the "restoration" - haha!