Details about Armor + Kevlar
The primary advantage of a full armor set is the protection it offers from direct headshots, which are otherwise lethal from many weapons in CS2. However, it does not protect against headshots from the AK-47, Desert Eagle, AWP, R8, Tec-9, Five-SeveN, AUG, M249, SG 553, or SSG-07. A helmet won't prevent death from headshots with these specific weapons.
Having armor and a helmet does not impact your movement speed, but when you take enemy fire, it greatly reduces the disruption to your aim caused by being hit.
The cost of purchasing a full armor set, including a helmet, is $1000. If you only need to add a helmet to existing armor, the price is $350. However, if your armor's durability is below 100%, you will need to pay the full amount to restore the armor and add the helmet.