In general, your best bet with militaria is to leave everything AS IS... the very most you should do is wipe it down gently with a dry cloth to remove any dust. Put it up on a shelf and admire it.
Your helmet survived ~70+yrs as is, so why add a repro decal? Some of the repro decals out there may look authentic and they may look cool, but not every helmet had to have decals on it. It looks like your lid never had decals in the first place, and is 100% authentic as is, so why change it?!
Plus, adding decals will raise red flags and discredit your entire helmet in the eyes of other collectors...
In terms of what you paid for it, I think you did okay. I'd pay about the same for it.
Its a nice lid, dont put fake decals on it. Also the price seems decent.
Thanks everyone for your replies! I will leave it just as it is, and put it on display.