After you beat the main game once, you can select "Load" on the main screen and load the game you have already beaten. Do this, and you can buy more guns from the merchant if you choose to. If you do, you will spend some of the money you had when you beat the game. After starting the game, you will have everything you had when you accomplished the first game.

It is okay to save. If you choose to load the profile that you beat the game with and start a new game off of it, all of the money will still be there.