You will need more than 2,500 currency to acquire all upgrades, so if you plan on completing this game this will come towards the end of it.
This popped for me about 200 gold away from buying the last upgrade.
As far as finding currency, the brown chests will award you the most. If your wallet is full DO NOT open brown chests or you will waste the currency and end up having to farm it during end game. Next to brown chests, blocks that you can only break with the downward strike are the next best thing.
I'd suggest upgrading your wallet asap so you can hold more currency. To upgrade your wallet (2 upgrades) you'll have to find Rampelkimpers.https://steamcommunity.com/sharedfiles/filedetails/?id=81078...