If you play a card and play darius does darius enter ready?

Yes. If you play one card and then play Darius as your second card of the turn, his ability triggers and readies him, so he ends up ready.

Both versions work this way: Darius, Trifarian readies when you play your second card in a turn (and he counts himself as that second card), and Darius, Executioner readies on play if you've played another card earlier that turn ([Legion]). Units enter exhausted as normal, but the trigger then readies him immediately — this is also true if you play Darius first and your second card later, since the Trifarian trigger fires whenever the turn's second card is played. It only fails if Darius is your third (or later) card, because the second-card trigger has already come and gone (Rule 419.4.a).