You do not.  Lighting comes with all of the Salesforce clouds.  You just need to enable it and roll it out to your users.  This isn't just a matter of turning it on, though.  You need to plan for the rollout.  Use the Lightning Experience Transition Assistant tool under Setup to work through your rollout plan.  

Although if have a brand new Salesforce org, it will default to Lightning so you won't have to roll it out, just train people with the Lightning interface right away.