Todays Military Surplus is Tomorrow's Militaria...
Mil Suplus is CHEAP, Militaria gets expensive.
I've collected Militaria for a long time. WWII, German, British and American.
The stuff I got as a kid in the 80's or earlier on the cheap is now Crazy expensive. My Grandfather used to tell me that he could go down to Montgomery Wards (the old department store) and buy Britsh Enfields and German Mausers out of a barrel for $15 a piece for the good ones and $10 for the less pristine ones.
So, Get what you like now and later down the road it will be worth something. Some things get more expensive others don't. My German Helmets all got more expensive, belt buckles seem to have hit a ceiling and aren't worth that much...and Daggers are off the chart, as well as Samurai (WWII NCO style) have been really hot.
If you want WWII stuff, more and more personal collections are coming available as that generation passes on. It is a sad reality. Unless there was someone in the family who was interested in the stuff, it ends up in Garage Sales etc...even in the Ebay world we live in, there is still a generation or two who will "Garage Sale" it....
The Vietnam Stuff I got as kid for pennies...is now picking up. I didn't buy the stuff as an "investment", more just something cheap to play "Army" in as a kid.
Oh and I wouldn't actually breath in that gas mask if it (or the filter) is too old....learn from the mistakes of others!!!!