I am thinking of buying a used PC. It is a Dell with Windows 7 currently installed. When I get it the hard drive will be totally wiped, there will be no OS. If I buy a repair/reinstall Windows 7 CD will I be able to reinstall it on this computer?
You don’t need to purchase a CD if you have a product key.
If you are using the sticker’s product key, make sure you install the version of Windows that the sticker goes with. There’s more than one version of Windows 7 (home edition, OEM, professional, etc) and the product key is tied to the version.
Since it’s a Dell, you can probably order a system restore CD for a few bucks from Dell. This will put the original version of Windows with all of the correct drivers plus all of the original software that came with the machine onto the hard drive.