Useful weapons and upgrades
All of the weapons along with their alternate fire modes have been described in the manual, so I'll only focus on the three tools of destruction I consider best.
Plasma cutter (0 credits)
The weapon we find at the very beginning of the game should accompany us up to the ending scores. Try and upgrade it whenever you can, focusing on improving the damage output (DMG).
Maximum damage: 18
Maximum cartridge size: 20
Ammo: Plasma energy (1200 credits for 6 shots)
Line gun (9000 credits)
Quite an impressive range makes it a great weapon for fighting large groups of enemies. A single shot on leg level should effectively slow down charging foes. In the alternate fire mode (Space) the gun produces a small orb of energy that explodes after few seconds damaging everything around. Buy damage upgrades first (DMG), considering the beam width (WID) as the second most important.
Maximum damage: 27
Maximum cartridge size: 9
Ammo: Line racks (2000 credits for 2 shots)
Flamethrower (11000 credits)
This choice may seem controversial for some, as the flamethrower is rarely useful, but it's irreplaceable when fighting the small, crawling necromorphs. It is however, not worth upgrading.
Maximum damage: 1,5
Maximum tank size: 250
Ammo: Liquid hydrogen (1000 credits for 25 units)