I love my Raspberry Pi 4B. Love it. I've worked entirely online, 8+ hours per day, for more than two decades, and my 4B (the CanaKit Raspberry Pi 4 Extreme Kit, 8 Gb of RAM, 128Gb mcro SD card "hard drive," $139.99 at Amazon, not an affiliate link) is my "desktop daily driver."
Unless you require special/proprietary software that's only available for e.g. Windows, the Pi can probably handle your desktop PC needs, at least if you go with something like my "high-end" kit. Kits with less RAM and less storage are still useful, but if you're planning to use a Pi as your main computer, go big.
Most kits come with everything you need to get started: The actual computer, which is just a stand-alone circuit board; a heat sink, and possibly a fan, both of which snap on easily; a case to cram that stuff in; an AC power adapter/cable; Mini HDMI to HDMI cables to connect up to two monitors (you'll need adapters or different cables if you're running VGA, DVI, etc.); and a micro SD card with NOOBS pre-installed on it (NOOBS lets you choose which of several operating systems to run, including the Raspberry Pi OS and some others).
But most kits don't include one absolutely, positively indispensable accessory: