There are many ways to skin a cat. If you’re new to streaming audio, I’d suggest you first assess what you can do with what you already have; then buy the minimum you need to deliver the streaming service(s) you want. You can (and probably will) upgrade later.
Most of us already have a computer that is perfectly adequate for hosting music software and streaming services, as long as it has an ethernet port and a USB port, and as long your HiFi system is equipped to receive the USB output. Connect it all up, then start exploring the available music software (eg Roon vs. Audirvana), streaming services (Tidal, Qobuz, Apple Music), and perhaps Digital Signal Processors (HQ Player, Bacch4Mac). You may start encountering issues such as computer fan noise, klunky user interfaces, limited multi-room capabilities, or limited streaming content for the kind of music you like. If so, explore what might be available to address those issues.