Fun idea to also use the countdown as both a health bar and a currency; that leads to interesting trade-offs. I can imagine you could take this core idea further in a lot of interesting directions.
Small gripe: it's a bit awkward to use arrow keys to move in a keyboard-and-mouse game, at least on my keyboard. I'd recommend also allowing for WASD movement as well, which is kind of the convention for these sorts of games (although I can imagine left-handed people may appreciate arrow keys more, so ideally using either would be possible).