The weapons are (in no particular order):
1. Plasma Cutter
2. Pulse Rifle
4. Force Gun
5. Contact Beam
7. Line Gun
You must own all weapons simultaneously, meaning they must be in either your inventory or your safe. You can only carry 4 guns at once, so buy 4, move them to your safe and then buy the rest.
The Plasma Cutter is a free story-related gun, but the rest cost on average 10,000 credits each, and you must aquire the schematics for each gun before you can buy it. You will therefore need just under 60,000 credits to buy the guns. The earliest you can get this achevement is mid-game once you have all the schematics, but they are quite easy to locate with very little effort.
Note that for the "One Gun" achievement (beat game with only Plasma Cutter), you can buy the guns, but you can't use them. In fact it is better to just save, buy the guns and then reload the save so that you can keep your money. The achievement will pop as soon as you buy the last gun.