Just like with rifles- if you stick with something the military uses, you'll probably never have a problem finding filters, hoods, spare parts, accessories for it.
If you go with something commercial, you may or may not be able to find parts, filters, etc. long in the future.
Think US or a NATO country or Israel, think 40mm "NATO" threaded cannister.
Ideally you want something with a drinking tube, voicemitter and defogging valves. Most military masks will have those features, some of the newer civilian masks like the Israeli 4A's will have them.
M17 gas masks hoods can be modified to fit a wide variety of masks and are relatively cheap- box of TEN for $19. or less. You'll need more than one cause if you have to modify the hood to fit your particular mask, you'll cut up one of the other hoods to make the modifications.
IDEALLY, the mask you buy will have it's own hood. Another option is to get a protective suit with a built in pullover hood. Something like the British MKIV suits. Avoid the older MKIII suits with the "anorak" style top. Like a sweater, it has to be pulled over the head to remove. Think about it....... It's going to be contaminated, you really want to have to pull the damn thing over your head to remove it?? The MKIII was a dumb design.... However it's a cheap suit as the British gubmint literally dumped them. The MKIV is the more current version with a zip up jacket top- easier to do MOPP exchange with.
Obviously your aware that with Ebola a lot of folks are buying these items right now so availability can be hit or miss right now.
In 40mm filters, your looking for M96 or C2A1 for your go to filters- again IDEALLY. The M96's aren't cheap, but again remember what the hell you are buying these for!! LOL