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. The video below details this method;
Helpful tip: to get cash incredibly easy, replay the Last Resort section in the campaign (chapters 16 & 17) on insane difficulty. Takes about 5-8 Mins depending on skill level to get roughly 20,000 each play through.
Contract missions are the only way to go if you need a heap of money fast. Playing the normal game can be a way to make a decent amount of money, depending on how good you are at the game. The Last Resort mission, the very first part, on Insane, nets me only 10,000-15,000. Maybe I'm not as good as most other players but it's what I make anyway. On Contract missions, you do the one at the radar station, you can make a fair amount of money. Particularly if you use a weapon with the grenade launcher attached to it as the game gives you infinite grenades versus the 3 normal grenades it gives you. This with invincibility while in the Overkill mode makes easy money. All up I made nearly 50,000. You stop when you want as the Overkill meter fills up for every enemy you kill with the grenade launch