Rob, the easiest/cheapest is the 1.98L Kruse bottle at the VW dealer. Usually you won't need it if you do a lot of highway driving. City driving uses more DEF and sometimes requires a top-up before the periodic servicing.
Break-in usually comes pretty quickly, and as you know (I'm assuming from your username), you can get very good hustle with the torque without getting near the higher RPM region of the engine.
You'll love the D!
2016 GLE 350d - White on Black