New
#11
If you have used Samsung Magician to secure erase the SSD then it will be unformatted, the same as it was when purchased. Boot from the Windows install usb and it will be able to format it for you as part of the install. This will be the same process as when you first installed Windows.
Clean Install Windows 10
No, you won't need to buy a new key. In fact, you don't need a key at all. When you first installed Windows 10 the PC got a digital licence for the edition of Windows that you installed. This digital licence is stored on the Microsoft activation servers and linked to the unique hardware ID of the PC. You can skip entering a key when asked. Install the same edition as you had before (Home or Pro) and as soon as Windows can contact the activation servers it will activate from your existing digital licence.Also, do I need a new windows key or can I use the same one I used last time?