More often than not, all DAWs come with the plugins you need to make good mixes and ultimately do a little mastering. The best 'essential' is to learn what each plugin is, what it does to sound and how to use each plugin effectively so that your music does not sound amateurish. As far as mastering goes, I would recommend iZotope Ozone 5. Like I said before, this is of course after the plugins that came with your DAW are utilized.