I will answer the second question first because it's the easiest: practically all Unix-based operating systems are equally customizable.
Arch is a rolling release distribution, which means you can install it once and keep it up to date forever. I haven't used Arch as long as everything else so I can't speak from experience, but I've heard this can cause instability on some occasions, which means you'll need a decent understanding of Linux in order to maintain your Arch installation.
Debian is a traditional distribution, which means the programs you install on it are as up to date as the version of Debian you have installed. This keeps things more stable than Arch, but you'll still have to set aside a lot of time to upgrade Debian whenever you need newer software.
I think the only terminal commands you must know are whatever your package manager is ("pacman" on Arch, "apt" on Debian), and "man" so that you can figure out how to use programs without googling everything.
Linux Mint is probably the safest distro to start with if you're switching from Windows. The default desktop environment is the same layout as Windows (you can also install themes to make it look exactly like Windows), and there's a graphical version of the package manager in case you're not entirely comfortable with the command line yet.
I hope switching goes well for you :)