I'm trying to create something in Unreal and I'm not even sure if it can be done without a plugin. What I want to do is when the player shoots something, have a sphere spawns at the impact site. If this sphere overlaps a platform, I want the collision and material on that platform to change. The tricky bit is that I only want the overlapped area to change. How would I go about doing this?

You have to be able to manage everything in your scene on demand, so no static mesh batching can be performed or else you won't be able to modify the object. Everything else can be handled by a simple Sphere Cast to get everything in a certain area