You will earn a total of 38,000 P.E.C. points. Only 26,900 P.E.C. points are required to obtain all the upgrades, uniforms, and camouflage variants. Which leaves you with 11,100 P.E.C. points to spend on the various unlockable weapons and uniforms, such as the Collector's edition Shadow Armor, the MP7A1, SCAR, et cetera.
Naturally, you earn P.E.C. points by completing P.E.C. challenges in offline, or online play.
DLC/Preorder/Uplay exclusive weapons DO NOT count towards this achievement. You do not need to upgrade them to get it. Big thanks to Kung Fu Riki for figuring this out.
One last random bit of info here, you'll retain all your weapons if you use scene select.