This is just a matter of collecting enough money/credits. I chose Luck to level up first as this then gives you more money from the boxes you loot as it's level increases.
It also helps if you can choose the Hacker, when it's an option, so you can access all those purple loot boxes and locked rooms too.
I kept playing from the start and once you've managed to complete all the levels, your loot will increase from playing the Overseer level 2.0 etc.