Designated healer?

Priscilla or Clarine have the most potential out of them, one them would serve you well

I think you should just pick one of the horse rider. They are slightly better as healer due to their greater mobility. And then choose the one you prefer between the two or the one with the stats you like more.

I really recommand using rehabilitate as it's probably the best healing ability.

Since you don't have Elise, go Priscilla

Personally I rotate them. You don't need to optimize healers most of the time (just inherit rehabilitate and a good buff). Rotating allows me to acquire more hero merit (three of my healers have hit 2000) and you'll likely want multiple competent healers for the Tempest Trials.

Clarine at +speed is the best for being able to survive enemy attacks because she will get doubled much less frequently than any of the rest. I'd say make her 4 star at best and make sure you give her Rehabilitate since it's the best heal and then Hone skill 3 and maybe Imbue to maximize heals.