That said, I'd probably check out a laptop in general if I were going to buy one to see if anyone has major problems. I was bitten about 5 years ago with one where the touchpad was too sensitive, no matter what I did.
But if you want to save yourself potential trouble, it's a good idea to research beforehand to make sure there's no obvious compatibility issues.
Also, these days various vendors sell machines with linux pre-installed so that's an option.
Best general advice I can give is to stay far, far away from anything with switchable graphics, and if possible anything that requires proprietary drivers.
I have one of those machines. It's from Dell. Hooking up an external monitor doesn't just work. The scaling across the Ubuntu UI is all messed up.
Is it still necessary to research whether a piece of hardware is supported on Linux, and how well it's supported, before buying the hardware?