Watch your ass when shopping for surplus masks.. There are a number of M-17,and it's subsequent upgrades, that look good, but unless you find good, and proper filters are suitable for Halloween use only..
The others? Well, make sure it has a filter that is interchangeable with whatever source of filters you have.. BE aware, that some filters are for training use only, and won't protect you from anything stronger than a fart..
Also, if you get one, it tests good, and has spare filters and parts, wear it. Put the damned thing on, and just do routine things with it. Hell, SLEEP in it.. If it is just too uncomfortable, claustrophobic, or you suffer respritoary problems with the partially restricted airflow, then find a mask that will work. Consider that you are buying something upon which you may put your life.. What IS your life worth?
I won't push one kind, or manufacturer. The fitting of a full face respirator, and how it performs is as much a personal decision, as the buying of a handgun for personal carry..
Good luck, and hope we DON'T need Gas Masks..
Meplat.