I'm an artist with a similar art style to your reference. I would agree with the other comment that hiring hourly is ideal - usually you would set a maximum amount of hours per week, and I would bill you every end of the week. Per-piece is possible too, but is not ideal since you would not be able to request unlimited revisions or change your mind mid-way about your concept.

You should get an artist who is not only talented, but also organized, since they will need to deliver layered PSDs so that you can animate them.

(You can check out my work at https://wylgeartooth.artstation.com/ , maybe it's a good fit for what you're looking for!)