I wouldn't recommend wearing a secondhand helmet unless it's never been used, and even then, you can't be sure it hasn't been dropped and messed up. I'd personally just go ahead and buy a new one- Ovation has some comfy, reasonably priced ones.
I think it’s way too risky. I always wear my helmet and I have always prefered spending more money on my helmet than on any other equipment. But again, it’s up to you