Since the Plasma Cutter is more than enough to get through the game, you really don't need to worry about buying a bunch of the weapons on the first play through. As long as you look through every room you come across in the game, you should easily find the schematics for all the weapons.
You'll have access to a store right before the final boss of the game, so you can spend the rest of your credits/sell unused items to pick up the remaining weapons if you wish.
After you complete the game, you'll be given the chance to save a Completed Game file. Loading this will start your game over, but you keep all your inventory/credits/suits. As a bonus, you'll also receive 50,000 credits. This bonus is enough to buy up the remainder of the weapons with some to spare. You'll have to move some weapons to your safe as you're doing this because you can't have more than 4 in your inventory at once.
The first store you'll come across in the new game is at the tram station at the end of Chapter One.