I only have three comic projects under my belt, so there are probably more experienced individuals out there. I know @zenia was (is?) working long term as a comic artist for someone.
But your best course of action is to sit down yourself and figure out how much of your story you want into a comic. You can't jam a lot of action into a single page, and comic page counts rise a lot faster than clients realize. It would be best to further break down your project into scenes.
For example a recent client provided me a four page story, and that translated to about 7 pages of comic work. That was with us trimming some things out/ cutting dialogue.
Expect to pay between $150 - $500 a page depending the artist, and I would advise against going with just the cheapest person. Watch the artist. For some comics are very daunting, so I would avoid paying all up front unless you really trust the person to get to work. This is assuming you don't intend to make money from it, but if you do some may charge more if you intend to print and publish.
How you want to commission each page or in batches is up to you. Personally, from experience, doing batches is easier to ensure everything flows together. The client can then sit down and review a set of pages to make sure they like how the pages and dialogue flows, and characters look consistent.
Before you commit to anyone I would suggest to shop around and ask questions. Make sure how someone works is something that makes you feel comfortable. Do they offer edits? When do they offer edits? Do they offer preliminaries? Do they do scripting for you? (Or do you have to provide scripts.) Do they take payment plans? Ect. Don't be afraid to ask questions. Someone who has a lot of experience working with large, long term projects will understand that clients are putting a lot of faith in them.