Complete the game once,and sum new costumes will become available at the beginning of a new game. Leon s kitted out in his S.T.A.R.S.uniform, Ada will wear her Assignment outfit, and Ashley will be sporting the full Britney Spears kit.
If you want to make a new game after you kill that damn Saddler, you'll be given the option of choosing Professional Mode, which is a more difficult game option, (trust me!). Enemies will take a fair amount of more damage, so you'll have to go easy on the ammo.
If you bothered saving the game after beating Saddler, then you load it from the load menu, you'll be able to start a new game, but will keep all of the weapons and items you had on you at the end of the last one.
On play-throughs of the game, you'll be able to buy both the Matilda and the Infinite Launcher from the merchant. The Matilda is an automatic pistol, while the Infinite Launcher is a well... a rocket launcher; it'll sting your wallet a cool million pesetas to buy it, but it's worth it. The Chicago Typewriter and the Handcannon can both be unlocked by playing through the two mini-games that are unlocked at the end of completion of this wonderful game.