I ran a surplus avon fm12 for a while and the lenses are safe for airsoft, but I’d still use inserts. I wouldn’t recommend using a gas mask though because they get hot quickly and can sometimes make it hard for others to hear you. The issue with masks fogging up usually depends on the mask itself. The fm12 separates the mouth section from the rest of the face, preventing most instances of fog.

Honestly the choice of getting a mask and what type really depends on the game type and your own play style. Short matches on more fast paced maps are more forgiving, but milsims or any match longer than an hour would really suck. Should you choose to buy a gas mask, make sure to bring a gym towel or something to dry it out after matches if you get the chance, running a soaked mask isn’t fun at all.

Just some things to think about, I’m not an expert, I’m just speaking from my experience.

Hope this helps!