I'd prefer to save a bit of cash
Who wouldn't? But anyone who can afford to indulge in the expensive pastime of flying certainly has sufficient money to afford proper kit.
I don't recommend penny-pinching on ex-military stuff, especially as it's ~50 years old. After all, military surplus gear was almost invariably declared surplus because it was worn-out, or judged to be obsolescent. Proper fit is also a concern, particularly if you're shopping via eBay. I doubt that you'd be happy with it.
I haven't seen all of the different soft helmets available, but I am familiar with those manufactured by
Perrone Leather in New York. They make a quality product.
I've been doing a fair bit of flying in a slow (50kn!) non-aerobatic machine with a front canopy, four-point harness and a cage structure around the cockpit, but no doors.
What kind of aircraft is it, btw?