TIP: when starting a new game. visit the base first. you can buy better guns and upgrades from the merchants there. personally i think the lasgun is the most worth-it gun you can buy at the early stages.

SPOILER: you can buy really cool guns from the NPCs in the powerplants or crashsites after you've cleared them. start out by buying a reload upgrade and a bandolier upgrade, then buy the gatlaser and stock up on ammos. once you clear any of the sites above and buy the really bad-ass weapons, the game gets really easy.