Both Sabina and Merlin are great in your situation. Merlin would make it easier to get through tougher boss stages in challenges and events (basically resets their mana). Sabina is a healer - and it’s great to build a base of healers of each color. She heals more than Boldtusk and removes buffs (but Caedmon/Sonya remove buffs also).
You probably wouldn’t regret either, but I’d lean towards Sabina. I have maxed Merlin and find him a bit too squishy for his average mana. When I take him to raids, he often won’t fire before he’s gone. It’s a bit of a downside (but I am raiding in diamond, so it’s quite strong opponents)
I don’t have Buddy so it is hard for me to compare him to Caedmon, but Caedmon is great, fast sniper and it’s supervaluable to have these - but his special overlaps with your Sonya’s, and Buddy’s is unique.
If you have Wu Kong, I’d recommend maxing him over any other yellow there is in the game.