
Once you have completed the story for the first time on any difficulty you will unlock the Extra Content Shop. You can access this shop through the main menu under Bonuses. Character and weapon models are purchased through this shop.
The Extra Content Shop uses a type of currency called CP (Completion Points). CP is earned by completing challenges throughout the game (many of these challenges are the requirements of achievements).
There is a total of 76 character and weapon models that are required to be purchased for a total of 8,900 CP. You are awarded 1,000 CP for purchasing all 76 so you will lose 7,900 CP total.