Well, I would focus on getting a perfect SU/CR unit if they are useful to you in the long term. Units like Fran, Liselotte, Waltz, Odette, Lucille, Shiho, Rinbell, Juno, Chloe, and Rika are all good to raise. The others are your choice. You can sell them for 3 RC each or use them to raise other units will their platinum rarity experience.
The new event unit is a black unit so it will be an item collection event. As of now, black units only get item collection events. And the event will start this Thursday.