Yeah, some essences are really easy to pull abilities from and others you have to get more creative with. So you can definitely think outside the box for someone like Don Quixote who has no magical powers in the original. Even pull from anything that has to do with the story from newer canons (I can't think of any tho).

Well, I don't know of any existing abilities that mix up medicine with poison but you can definitely run with that! The limit is your imagination!

Hahah he probably shouldn't have fire magic but it could be something he works up to, yeah. Magic steed could also be a thing, like he whistles and the horse instantly shows up no matter where he is - unless it's like underwater or something.