You can assign the license directly on the host without vCenter. Just open up the vSphere Client and connect to your host. Select the host then Configuration->Licensed Features, at this screen you can click on Edit on the top right side and assign the license key there. To answer the second question, the only time you need to purchase and install vCenter is if you plan on utilizing the features of managing your hosts through vCenter.
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James B. | Blog: http://www.vSential.com | Twitter: @vSential
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