Buy all Weapons, Tattoos, Masks and Outfits
This will likely be the final achievement that you will unlock as you will need to do multiple playthroughs to earn enough money to purchase everything. As well as the 30 guns and 19 outfits detailed above, you will need to purchase 22 tattoos and 56 masks.

19 of the tattoos are readily unlocked from the beginning, but you will need to attain a certain rank to unlock some of them, before you can purchase them. You will need to reach at least rank 23 to unlock the rank specific ones. The final 3 tattoos that you will need to unlock and purchase are all unlocked after you have completed the game on any difficulty. Each tattoo costs $1,500.

38 of the masks will already be unlocked from the beginning, but as always, you will need to reach a certain rank to unlock some of them. You will need to reach the maximum rank of 25 to have unlocked them all. Besides these there are 17 that you will unlock for completing the game on any difficulty and 1 final mask that is unlocked for completing the game on Insane difficulty.

After you have purchased every single purchasable item, you will unlock this achievement. If you happen to have bought the Overkill Edition of the game, you will have access to 'Contracts Mode' where you can earn a lot of money in a short time.