Simple Question for you pros

I am new to Parallels. I am about to purchase an iMac with Parallels but know that I have to also install a Windows OS separately. I do not care what Windows OS I use but am wondering do I really have to buy a full Windows OS program or can I simply purchase an OEM disk somewhere such as ebay and use that to run Parallels? Will any Windows OS program work such as Windows 2000? Thanks.