Head: Leatherneck (VvV)
Torso: Hollow Point (BTR)
Arms: Leatherneck (VvV)
Waist: Unsure (RAH?)
Legs: Leatherneck (VvV)
Mercer's is one of the image updates I enjoyed from Devil's Due. For my interpretation, I started with the Defense Mech Leatherneck and the Hollow Point torso: I felt the high collar would build on Mercer's tough guy attitude.
I sanded off the left-hand pockets to place the pistol, then just set everything in his colors: red and blue. Sure, the red would make him a target -- but in my Joe-verse, Crimson is a level of armor that a trooper must earn by acquiring on his own, it is never issued by commanders.
So, in my mind, Mercer's got himself the chestplate from a Crimson viper, whose body armor could withstand direct hits. Either he acquired it before his defection, or he took it as a spoil of war after joining the Joes. Either way, Mercer's fully aware of its body armor capabilities, thus why he continues to use it.