So, if it comes preloaded with a Windows OS...you'll buy another copy of Windows and effectively pay for the OS software twice?
Depends on how much you value your time and frustration dealing with the crapware that came with the OEM installation. You can often buy hardware without Windows installed, save the Windows tax, and install a clean copy bought elsewhere.
When you use Windows, you must be prepared to pay, and pay some more, and then spend inordinate amounts of time fixing and cleaning and repairing. Or risk malware with "cracked" versions to save a few bucks.
Personally, I prefer to run my Windows in VMs (virtual machines) hosted on either Mac or Linux workstations. That said, I have three Windows installs "on the metal" at home, dual booted.
No, you already have the OEM license generally when you buy the PC. Just pay a little extra and get the DVD media (they don’t include that, only their customized “restore” media) so you can install it clean and without all the garbage the stick in it. I don’t think it’s right, but they do charge extra for the windows media. It’s a manageable cost though... usually 10 or 20 bucks or something like that. The install key is on a sticker on your computer, all you need is the install media and use your existing key from the sticker.