I think it depends on what the seller offers and how much your budget is.
If you’re not sure on what style or aesthetic would work best, perhaps spending 5.00 on a few different sellers for a basic mock-up, might point you in the direction of who you’d want to work with for the whole book.
After you find someone, since it sounds like a larger project, perhaps you might need custom pricing-- in which case you can message them and explain exactly what you need, your timeframe for completion, etc, just be specific and realistic. If you’re comfortable sending in the entire book, I’m sure that would be beneficial for the illustrator to work with, as it would seem doing it piecemeal might diminish the overall feel and momentum.
But who knows, each seller is different. Just keep an eye out for what seller’s styles appeal to you, get in touch, and see what they can offer you.