You currently do not have a Healer? If not then go Blood Knight for Victoria, she doesn't heal very much but it's something nonetheless. She can replace either Vincent or Shizu, depending on who you think is under performing in the arena.
No, it's better to invest in someone who can last for the long term. Bulking up Claude is just gonna waste your resource, he's not that good anyway and he'll get killed easily. You'll get a 5* potential Healer eventually, just a matter of when.
For now a Blood Knight Victoria as mentioned should suffice.