So I was browsing steam and I saw this was on a heavy discount.
It looks like the base game is 4.49 and the Omnissiah edition is 5.24 I'm not sure what is in the Omnissiah edition, it looks like just sound track and art. Does it effect the actual game at all? Is it standalone to play or do I need the base game to play? If it is just soundtrack and art will I be missing anything by skipping it.

Then there is a bundle Complete Edition which is the Base game, Omnissiah, and Heretek for 12.31
It looks like Heretek is an add on and has more game in it so I would want it. Is that correct?

But then Steam has a link to get all the DCL which is Omnissiah edition, Herektek (no plus?), and Mechanicus: complete soundtrack
For 13.47

EDIT: Based on what was said I decided to get the Complete Edition. Lets purge some xenos! I realize under 20 bucks it doesn't matter much but its annoying I can't easily figure out which options and bundles come with what.

Thanks in advance