From personal experience, the Chromecast 4K on its is very limited on free space. That means if you want to install more than a handful of apps, you will need to resort to a USB hub, most likely a powered one, to connect a USB flash drive or SD card in order to expand the usable space.
If you think that’s not a problem, the Chromecast is a very capable device. I won’t put it on the same level of performance like an Apple TV 4K, but it can do everything I throw at it, albeit in some slowness from the UI from time to time.
I’m no expert in the matter but from what I gather here Steam Keys and in other comments around the web, Valve allows free key “generation” for resale in other stores.