For regular raid defense, no you don’t necessarily need a healer, as long as the team is quick and strong enough to take out the opponent.
You have some options for tanks; Hel, Vivica, Athena. The only reason to do Vivica over Poseidon imo is if your alliance uses same color Yellow war tanks.
If this is for defense, I think Kadilen’s higher stamina and defense against special skills will mean she lasts a lot longer than Margaret. But I think Margaret is the better hero overall… she’s much better at titans and for green stacks with her high attack stat.
So I’d be tempted to ascend Margaret first myself. If you do she can work fine flanking the tank.
What I would do:
Poseidon | Hel | Athena | Margaret | Marjana