I would think some shader "layers" and/or "masks would be a good way to achieve this. You could use some bump and even "draw" in the height changes as well as the texture color. Then you could do another layer/mask for the grim and surface imperfections. This would be worth looking into.
I would recommend texture painting! If that's new, I'd suggest following along with this course first: https://cgcookie.com/course/modeling-texturing-shading-a-treasure-chest-in-blender-2-8
