I just bought the Omnissiah version, but I saw the Heretek dlc to buy. But this dlc has two versions: a standard one and another with artwork and stuff. I was interested in this more complete one, however it was not clear if in this version, I would be buying the base game again. Could anyone clear this doubt?

Same, I had to wait for big sales as it was not clear.
Buy the "+ Edition" of Heretek DLC, and you'll be good for the more complete game
(if you already have the Omnissiah Edition)