Need Tumblr Advice

Hey guys new here to the forums, hardened marketing veteran. I have some money to spend courtesy to Harlem Shake and Gangnam style (20 million + hits) and am looking to set up a Tumblr blog. I have my niche, unexplored of course and a domain in mind (ideal domain is taken!). I am using Tumblr as a primary and will be using my standing adsense account with it. Should I set-up the domain or just keep to Tumblr's default domain. Thanks guys and expect me on here a lot more!