I've done a few paid comics, so I got to experience some of the bumps in the road you can encounter.
Most important thing to consider is that a large project like a three-digit pages of comic work is a huge thing to commit to for both parties. You will need an artist that behaves very professionally.
Since you're going for a long term deal... First make a selection of artists that match at least your quality and style minimums. Then study them carefully to narrow the group down to the most professional ones when handling their business.
Celestina's points seem pretty good, so follow that and then talk the terms of the deal with the artist you chose, see what they expect or demand from you through the duration of the deal.
It's best if you define all the aspects of the deal at the start, set soft and hard deadlines including what happens if they start being broken, how and when will be the payments done and what happens if you fail on making them, what are the outs like what happens if they or you want to stop the thing halfway, and what the posting/publishing rules are. If you plan to profit from that comic your artist needs to charge you for commercial purposes.
I'd suggest to work in chapters of 15-30 pages if possible(maybe a few more if it's only inks), so there are good stop-resume points along the way if you need to change the artist, so your thing can continue without a huge impact from the eventual changes in style.
Basically the more details of the deal you can address ahead of time, the safer you both will feel as you work together, instead of having to discuss these things as they pop up when the whole thing is already in progress and causes friction and stress.
Be fair and honest with your artist while they do the same in return. Be ready to be stern and hold your ground if the artist demands something that you really don't want to do their way, and be ready to concede certain aspects of the deal that will be pretty important for the artist to work their way.
👍 Good luck.