For those who wonder what to upgrade, disregard damage past lv. 4 since it takes a ridiculous amount of damage upgrades after lv 4. to have an impact on the game. if you want to kill the fat zombies faster, just get critical to get the chance to one-shot them instead.
There is a bug, where if one saves the game, and reloads it, all the money nods reset. Don't know if this is discovered by anybody yet, but it is really helpful, especially on the last mission on hard...
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Thanks for commenting :)