I want to throttle the internet speed of one PC in my network and not sure if that task could be easily accomplished, without buying something expensive, like a professional equipment (for example a managed switch) .
Concrete, I have an ISP 100Mbit download- and 40Mbit upload-speed and the dedicated PC should only get modem speed via WLAN (symmetrical 56K). Does anybody have an idea how I can throttle this connection, perhaps a setup based on a Raspberry Pi / Pi-hole or a similar platform like an ESP...?
You could probably configure this on said computer; how would depend on its OS.
Your router (or separate WiFi Access Point) may be able to limit the bandwidth for specific connected computers by IP address. I know my Draytek router can do this, but you will have to consult the manual of your router to see if this will work for you.
You could, if you really want to, build something yourself that does this, but it will be complex; I wouldn't go there, at least not until I was sure my router, or even a replacement router, could not do this out-of-the-box.
If you don't mind using the sort of hardware that can run Linux, maybe you can find software that does what you want, and you certainly can find router software that will run on it, and that router software may well have the bandwidth-limiting option you want, or maybe you could add it yourself.
You would be adding an extra router to your network, but there is nothing wrong with that. Developing the whole software needed from scratch without router software to start with will still be complex, though.