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The Chromecast has a built in DAC. You plug in an amp or monitors, connect to your home wifi and it just works. It has TosLink digital output as well as analogue audio, so you can connect it to a swanky audiophile DAC if you so desire.
I prefer studio monitors, but there are merits to hifi gear. Studio monitors don't require a bulky external amplifier, but you do need to run a power cable to each speaker. They're designed to provide a neutral and accurate reproduction, while hifi gear tends to have a slightly hyped frequency response. There's a degree of personal preference involved. If you're spending a decent amount of money, it's worth heading to Richer Sounds and your local music tech specialist to hear stuff in person.
Hard disc audio players work as a stand-alone system, controlled via a traditional remote control or a special app. You plug the player into an amp or powered speakers, as you would a CD player. Some of them just play music stored on the hard drive, some can connect to streaming services like Spotify and Tidal. It's not an option I'd recommend, because they're very expensive and often have clunky interfaces.
Long USB cables require active repeaters every five metres. If you're using a wired connection over a distance, it may make more sense to use a long audio cable.
Any old PC is fine for audio streaming. You could use a Raspberry Pi as an audio player, although it's a considerable faff to set up. You'll need to install Linux, then figure out some way of controlling it. I wouldn't recommend it if you're not an experienced linux user.
The big advantage of Chromecast Audio is that it works absolutely seamlessly from your other devices. You just press the cast button in a supported app and the music plays through your speakers. You can stream audio files from your PC using Plex, but you can also stream things like podcasts, Spotify or iPlayer Radio directly from a phone or tablet.