As a new Marauder I'm wondering which of my stats is the more important one for PVE. Mastery, Power, Accuracy....help? lol thanks

I got a bunch of Cartel Market Certificates from the Nightlife event but can't spend them, as I don't have the rep. How do I get it?

Thanks for the info. Much appreciated

If I chose healer in this screen can I go back and change it? I want to be a healer for end game content for my guild but I solo a lot so I don't want to hurt myself. *Edit due to PC police*