I tested Heavy Armor, Special Force Helmet, and Tactical Protection. They do not cost much, does not not slow you, and give a lot of damage reduction. Take them in above order as soon as possible for commander, then later for girls. This should make this game much more easier for you.

Hello. Some tips. It is more tactical game (inspired by SWAT simulation series, but much more easier), so it is better to play it cautious (more stealthy with eye on unexplored corners and doors), not running ahead and hitting everything like rambo style in most fps games. There are medic kits that can fully heal you (key "5" and left mouse button), get about 3 for you and 1 for each girl on every mission. Use cover behind things with duck (key "shift", but it would be better to change into standard "ctrl" and "shift" for running). Use lean (key "Q" and "E") when you are behind cover or wall or door. Use aiming while shooting (right mouse button) to improve your accuracy. Aim for head, if target is close enough. Reload (key "R") after you take down target or targets and it seems clear for a moment, even if you still have half magazine. Increase mouse sensitivity in options, if it is too slow (I use max). Position your girls (key "Z" and "C") in proper tactical situations or to move them ahead if you are low on health with no medic kits. Help girls when they are shooting or especially reloading, so you keep them alive to support you longer when you are under fire from more than one direction. Remember to repair your armor before starting a mission. Buy better weapons when you have more money, because they do noticeable difference (I bought helmet for girl and then collected money for DE Hawk pistol with extended magazine that allowed me quick take downs with 1-2 shots).