One more tip, replay missions a bunch. You get paid the same every time and if you're stuck on a boss it's probably because you have a build that sucks for it.

Doing a bit of grinding on some of the earlier missions gives you tons of money to buy parts and if you die during a bossfight you're allowed to refit before you respawn. So don't worry about having a perfect build for the boss if it sucks for the early part of the mission.