Great art style and I like the idea of using the timer as a currency! Movement is fluid as well!
The pistols not having a cooldown and dealing low damage made combat feel like I had to spam the mouse to play effectively. Maybe give it a short cooldown but make it deal higher damage to stay in line with the other guns?
Keeping the upgrades next to the wall also makes it risky to buy things as it is easy to get cornered and stuck in a group of enemies.
Also, there is no way to confirm if you are selecting an upgrade or not. In my first attempt, I walked into an upgrade and immediately died because I did not know that my time was the currency. You could tie buying upgrades to a key press while nearby instead of just when entering the trigger area.