Alright, so there's a lot here... Linux is a subject near and dear to my heart so I'll split this into points.
1. Because of the CPU architecture and several other factors, Pis were designed
TM to run Linux. However, a quick google search shows me that it is technically
possible to run a full Windows 10 installation on a Raspberry Pi, but according to
this article it seems like an absolute nightmare. I don't know enough about MacOS to comment but last I knew it's breaking Apple's ToS to install it on non-Apple hardware...
2. If all you need is a browser, a Pi running Linux is a decent option with a few caveats. First, while the Pi platform itself is pretty solid, they do run off of SD cards, which can be finicky. This page of tested SD cards is pretty helpful:
https://elinux.org/RPi_SD_cards Also, remember that the Pi hardware is pretty limited. My experience is limited to the Pi 3 model B, so running a graphical browser like Firefox/Chromium is pretty laggy and doing anything RAM intensive like streaming video isn't great. However, the specs of the Pi4 are improved especially if you can get the model with 4GB of RAM.
3. If you just want a cheap computer, my recommendation would be to keep an eye on eBay for Lenovo ThinkPads. They are super durable and reliable and are likely to be an instant upgrade over a Pi while being comparable in price. Also, I don't know what kind of connections you might have, but it would probably be worth asking people you know who work at large enterprises if they have any spare computers they are looking to offload. As someone who works in IT, the turnover rate for corporate hardware is pretty high and it's not uncommon to see businesses donate/recycle dozens of perfectly good computers that are 5+ years old. Pis are definitely worth considering though, especially if you like techie projects.
