It looks like we'll soon be welcoming a lot of new Linux users here

Go slow, pick an easy distro like pop OS and take it easy.

If its for gaming, even https://nobaraproject.org/ is great as it has a lot of gaming optimisations.

Remember, Linux with a GUI is not more complicated than windows with a GUI, you have just spent your whole life learning the windows one.

Yes.

1. Back up your files.

2. When building the computer, go for generic middle of the road normal hardware. Fairly easy to do with off the shelf ATX PC hardware. Inside the case, this usually means look out for weird graphics cards or motherboards. I'll warn you that Asrock RGB lighting doesn't like to play with open source control software. Outside the case, pick a keyboard and mouse that don't require (much) in the way of configuration because Razer and Corsair don't publish their bullshitware for Linux yet.

3. Back up your files.

4. I recommend spending some time with whatever current hardware you have trying out a few distros in a virtual machine. Don't just look around and go "ah that's nice. ah that's weird." Actually use it to do your work. Even though you're running Linux IN Windows, try to use Linux to do actual stuff.

5. Back up your files.

6. Choosing a distro. Isn't really all that important, at least at first. Most of the meaningful differences are going to be in the Desktop Environment anyway. There's about 18 different GUIs you can use, from weird tiling window managers the hardcore nerds tend to like, to more Windows like experiences in KDE and Cinnamon, to more Apple like experiences with Gnome and Pantheon. Try a few out in virtualbox.

7. Back up your files.

8. Learn a little bit about the terminal. A lot of people hate and fear that suggestion, but it can honestly be fun. Wait till you see what the command fortune | cowsay | lolcatdoes. Learn how to edit files, run commands, install software via the terminal, even if you don't plan on doing it that way routinely. Mainly, so that if you ask the community for help, you're not completely in the dark when given a terminal command to run. Which is often the case; because "click here then here then there then tell us what it says" is harder to convey than "copy paste this command into the terminal, and then copy-paste what it says." There's a lot of cool stuff hidden in there.

9. Back up your files.

10. Have fun!

For windows you dont need to buy it, you can activate it using microsofts own tools, its on github (dont remember its name right now.)

As for linux, i would recommend openSUSE Tumbleweed, it is fresh, and the best part about it is that whenever you update the system, it creates a snapshot, so if the update had some kind of undesired sideeffects, you can just startup the old version. (These snapshots only effect the system's packages, your apps will keep their state iirc. My brother uses tumbleweed and he is very content with it.)

A very important one besides backing up your files is that you should more or less stop searching the web for applications to download and install. You should first try to use your package manager (read the wiki of your distribution to know which one it is and how to search for and install software with it). If you miss some applications from Windows, you could have a good chance to look for alternatives at sites like https://www.opensourcealternative.to/

Do not fear the terminal too much. I know, Windows tried to make it as dreadful as possible to use, but if you do not run sudo rm -rf / (deletes your whole filesystem), there is not much that can go awfully wrong. And you should only execute commands you understand. If something does go wrong however, it is a good thing to have a backup ready (I would recommend Pika Backup or Vorta, both based on the great CLI application borg)

For things that are not explained in the wiki of your distro, you have a pretty good chance to find a good explanation and even troubleshooting tips at the Arch Wiki (e.g. I use Void Linux and still search for most things at the Arch Wiki). It is also good to read some parts of the manual pages if you did not find enough information at the Arch Wiki (the command man is your friend) and the software's wiki page if it exists.

If you ever find yourself in the editor vi or vim, type :q and press enter to quit until you feel the need to potentially invest a lot of time in learning vim movements to increase your editing speed and you never want to go back. Use nano, micro or a graphical app at first instead and keep using it if you are not someone who edits text a lot.