As others have stated, it sounds like you've planned a lot for these. The only suggestions I would make is that if you paint these, you can lightly sand the outside of smooth plastic to roughen up the surface which allows the paint to adhere better. The paint tends to flake off otherwise... make sure to wash it off after, or you'll be painting over the loose plastic you just sanded.
If you're planning on these being out in the wild for years, a couple layers of paint wouldn't hurt. Also, if you end up using the textured paint (like the sandstone you mentioned), use 2-3 very light coats as the compressed air from the spray will "push" the flecks around that are still wet.
Make sure you let us know when they're placed, you might be surprised at how many people might be in the area. As a new cacher myself, I'd love to go hunting for them!