Be prepared and passionate, illustrate how are you going to use it, for what purposes, be clear that you're going to do it outside office hours, set limit on how much company resources you're going to need, is the CMS something you can self host on your own accounts or you're going to impact on day to day operations?

Is there any upside for the company?

There's also another aspect: will the data you create with this CMS belong to you or the company?