You can buy windows anytime you want .
Installing a dual boot adding windows to a ubuntu as initial OS may require some research
It’s something on my todo list, so I’ll add references as I get them
Here’s what I have so far:
I recommend the reference below:
The problem here is that the Windows installer tends to overwrite Grub bootloader or whatever making only the Windows installation bootable. But the good news is that your Linux partition is untouched, including the original bootloader and the other Grub configurations. So restoring or recovering Grub is just a matter of running a few commands on the terminal.