A docking station is just like an external case, except it's really easy to swap a drive in/out. So in effect it turns your hdds into massive flash drives. It'll only fit one drive at a time.
And internal drives are exactly the same as the drives used in the external enclosures. There same drive you buy with an external enclosure is also for sale just by itself.
The reason why you'd get a docking station is it would allow you to quickly access one drive, then swap it for another, etc. If you want more than one drive in at a time, you'll have to buy either another external case or another docking station.
If you really want to go the whole hog, getting a bunch of external cases would be better, because you'd be able to access all your drives at the same time, without having to change anything. However, that would be quite expensive...