The black plastic base is to allow the router to slide and not scratch a wood surface. There is a recessed "pocket" in that black plastic base plate made to hold porter cable type guide collars. I can't imagine using the router without the plastic base. I have an assortment of Lexan base plates with different shapes and different size center holes for larger router bits. You can buy extra black plastic base plates.
Do you intend to use the router free hand or mount it upside down in a table? Most table mount systems would remove the black plastic and mount the base to a plate of some type.
It is not unusual to need to cut a larger hole in a base plate to accommodate a larger bit, but I do think you want to keep the base plate as close to the size of the bit as possible.