Fully agree with Phil.
I use two 5V fans, sitting behind the rebreather tubes. The are mounted to blow at your face, and run from 3x1.5V AA batteries. Thus, they run at 4,5V, undervolting them slightly and running very silent because remember, all sounds inside a closed helmet are amplified ... by a lot. You do NOT want a loud fan there.
9V batteries can be used for stuff like the LED's on a helmet, but at maximum 200mAh running your fans from such a battery is not going to last a full day on a convention.
At 2500+ mAh, the AA batteries are much better suited for this.
Note : do not use rechargables for this kind of setup. Rechargable batteries generate only 1.2V per battery, so 3x1.2V is 3,6V. Most 5V will not start with just 3.6V.
I bought these fans on ebay, they costed only 6USD for 5 of them !
Another titbit : depending on used helmet material, you might consider not simply glueing these fans into the helmet. If your helmet is rigid on the inside, you're better off making a foam "base" with a large hole in the center for the fans so that it can absorb possible rumble of the fans, and glue that base inside the helmet glueing the fans on top of them.
On a foam-based helmet, this probably will not be needed.
A "quick fix" for visor fogging : silica gell beads. Not ideal, a fan system is always better, but it does work better than doing nothing at all.
Fans are the key to have a comfortable time inside your helmet. Certainly if you're walking around on a convention where the temperature will rise anyhow just by the crowds present, having a breeze of cool air around your head is the difference between feeling tired very fast, or be able to run around for hours and having a good time doing so.
Included : a look inside my helmets' fan system.
As for keeping armor pieces in place, velcro is the easiest way (fixating one part on your undersuit, and the other hotglued in the correct positions of you armor pieces. Another way is using magnets, but these have the disadvantage they will make it impossible to wash your undersuit in a washing machine if they're fixated on your undersuit.
Push buttons CAN work too, depending on what you're trying to fixate and where. For example, Push buttons can be a good choice for the codpiece on certain armors.
PS : Da ist gar nichts Falsh mit dein English. Keine Bange ! Viel Spass auf's Forum, und Hertzlich Willkommen !