Although we're still working, we can tell you some aspects of game mechanics.

1. Hunger and Thirst

Your mercs as people may feel hunger and thirst and it is difficult 

If the value of Thirst or Hunger reaches to 0, the character receives a -5% penalty to all checks for each zero value. The total of the total for this depletion can result in a penalty of -10% to all checks.

2. Tiredness

Tiredness mechanic is optional, so GM is decides when characters are tired.
Your mercs spend a lot of time on missions, so they become exhausted and tired. If character is tired it gets -10% penalty for all rolls.

3. Pain shock and instant death

If the player character gets 6 or more clear damage (damage that is not blocked by armor), then player must roll d100 to check Constitution  (CON) * 5. If roll is successful, then character is fine, else felt unconscious.
If a character takes 8 or more damage, deducting the damage the armor takes to the head or neck, he dies, but the GM may make an exception at his discretion.
If a character's limb takes 8 or more damage, he loses it. After losing a limb, the player must make an instant death saving throw and unconsciousness saving throw check using Constitution (CON) * 5.

4. Ignoring the armor

If the player made a successful throw using in combat a targeted attack, let's say the targeted attack was made in the head, then to bypass the defense (helmet) the player can make a shot in the face, if not taken. Similarly with body armor.

That were some of the game mechanics. Thanks for reading. 
You can ask any questions about game in comments.