So you removed the fireplace in its entirety and closed up the room, leaving only the chimney in the attic and roof? If so, I understand why online quotes are varied.

I'd take lots of pictures and take them with me to visit some local fireplace stores - they can steer you in the right direction.

If you haven't already done so, please ignore whatever you have heard/seen online and go see an actual fireplace professional. You can find one here: National Fireplace Institute professionals

I'm not surprised your estimates so far are "all over the place". I do this all the time and I would be hard-pressed to offer even a ballpark figure without knowing a lot more than you have posted.

Best wishes on your project!