You can install an OS on an external device (HDD, cards, flash drives, etc.) if your BIOS supports booting from USB devices. Otherwise, you'll need to purchase an internal hard drive.
That's all anyone can tell you without knowing the model of the laptop.
As for getting Windows for a discount, there's no legal or illegal way that I'm willing to discuss. If you don't want to shell out the money for Windows, I suggest you listen to Kuszmania. If you're not wanting to really use this laptop for gaming, Linux is an excellent choice. It's free, it's functional, and most builds come with loads of useful software... all free.