The easiest way to upgrade all 12 parts is to save up almost 100k and then delete anything thats not the first (Cheapest) gun and then just upgrade those.
The only thing I didnt like is that the game was too short and easy...I beat it within an hour and two of the units I used were the starters. Aside from that this is an amazing game 5/5
 "Fleeing the Complex"){kind=icon}
 "Bullet Heaven"){kind=icon}
 "UPGRADE COMPLETE!"){kind=icon}
 "Sonny 2"){kind=icon}
 "Sonny"){kind=icon}
 "The King's League"){kind=icon}