Linux

Generic answer for this is to get a refurbished corporate laptop. At least in here we have several companies which buy previously leased computers and give them a refurb (new hard drive, good cleaning, things like that) and sell them for pretty good price.

W, T or X series Thinkpads are pretty safe options, my T495 was 300€(ish) on sale. L and carbon are something I'd avoid, L (at least few years back) weren't built as well as T-series and X1 carbon doesn't have options to expand/swap out ram.

Contrary to what others here are saying, instead of trying to install Linux on something random, I would highly recommend getting a Linux-first / Linux-friendly hardware at first instance, if possible. If not, that's fine, you could just go for any random off-the-shelf PC, BUT you run the risk of certain things not working or perhaps not fully optimised. For instance, for laptops, one common issue is standby/sleep mode not working properly - either you may experience battery drain during sleep, or things like the graphics or audio might break after resume (or worse, might even crash). Or you may experience higher battery drain in general compared to Windows, or certain things like some Fn keys for brightness may not work etc. Certain sensors not working is another common issue, which may cause your fans to run at 100%. And if your system has an nVidia card, there's a good chance you may run into issues - if not during installation, perhaps after a system (kernel) upgrade. These are all just examples btw, just because you're buying something random doesn't mean you may face these issues, but it's something to keep in mind.

The general rule of thumb for buying Linux-friendly hw is: avoid nVidia. Both Intel and AMD onboard graphics are fine (for the most part - at least, they have a better track record than nVidia anyways).

There are some PC/laptops which come with Linux loaded out-of-the box, which would be the ideal Linux machine to buy. I call these Linux-first machines. Examples of such machines include System76, Star Labs, Slimbook Fedora, Tuxedo, Purism, Juno Computers and Entroware to name a few. There are also some laptops from mainstream OEMs which come with Linux, such as the HP Dev One and Dell XPS Developer Edition. You can't go wrong with any of these machines, as they're Linux-first machines and have been tested by the OEM.

Then there are Linux-friendly laptops - these are laptops which may not necessarily come with Linux, but have been either confirmed by the OEM in some capacity that they're Linux compatible, or they're widely supported by the community due to their Linux-friendlyness and popularity. Examples of these include the Framework laptops and Lenovo's ThinkPad series. You generally can't go wrong with these either.

For other machines, your compatibility can be a hit-or-a-miss. Some laptops may work fine, but might need some extra configuration steps to get it all working - eg Microsoft's Surface laptops need a special kernel to be installed (and some additional steps to be followed, depending on your distro), but otherwise work fine once you've installed all the extra bits. If Linux-first/friendly laptops aren't an option for you, you could go for something random, but you'll need to do your research (Google/reddit search etc) and see what their compatibility is like, whether everything works, if there's any issues etc. In fact, this would apply for most of your hardware purchases going forward. Eg, say you're buying a printer or some external webcam, you should check for it's Linux compatibility first before buying it.

Finally, if you want to run Linux on a device you already own - you can (generally) try it without any risk, using a Live USB (bootable). Just download the image, write it to your USB drive and boot from it - this will give you a good feel of things, and highlight any potential compatibility issues. Ventoy makes this a very easy process - just install Ventoy to your drive first, and one it's done, you can download multiple ISOs of various Linux distributions and just copy it to your drive (no need to install them). Then you can boot from the USB and select the distro you want to try and boot from it. This is extremely handy for trying out various distros, or just keeping some "rescue" ISOs handy. You can even boot a Windows ISO from Ventoy (and other OSes too, like the BSDs).

I also recommend getting a refurbished ThinkPads for their excellent Linux support, especially newer models like the X1 Carbon. No need to worry about such and such hardware not working on Linux. Avoid models with Nvidia GPU for less maintenance headache.

If your budget stretches to a new laptop, framework make laptops (light weight or serious game machines) that come with Linux installed.

If you have no budget some second hand high end Chromebooks can be converted to mainstream Linux (video)

The other comments suggesting old ThinkPads are fine, but they're heavier than I like to carry

You should really edit your post with what your budget is and what your requirements are if you want anything other than brand recommendations.

Pinebook Pro is rather nice, though it is low powered and some tasks will feel slow on it.

I'd say don't use debian because GNOME environment feels restrictive. No window snapping was my biggest turn off.

I use Fedora - KDE which is really comfortable to me. Others have said Mint Cinnamon is a good one to start with.

Whichever OS you choose, when you install it, for the love of God, plug in a network cable so you can use internet on the thing as it installs. This will save you much trouble during set up.