Hello, hidsmurf2000,
You can retrieve the index of a given static mesh by using the “Hit Item” output pin (int) from a hit result. Then you can use that to get the particular properties of that instance (such as its transform) after you have casted to the InstancedStaticMesh class from the hit actor.
See this tutorial for details: UE4 - Interact with Foliage at Run-time (Cut down trees, remove foliage, etc...) - YouTube
Also, if you are talking about accessing the index from within the material, there is a material node that will get a random number out of the instance index and that will help you to randomize your looks a bit.
See this answer: https://answers.unrealengine.com/questions/280115/is-it-possible-to-use-instanced-material-on-instan.html
As for passing parameters to the material instance of a particular instanced mesh, remember that they are all the same draw call and are sharing the same material, so you would be affecting them all.