It really depends how much of it you want to convert into the black color. For example, if you are OK with the interior being the same and are OK with bits of grey appearing in certain parts of the model then you wouldn't need to buy every piece in black. For me, I wanted everything to be switched over to black. I didn't want any grey pieces to show on the model at all. Even the technic beams that you usually wouldn't notice in the original color scheme will show up once you start converting the other parts to black. If you truly want a consistent all black look, I recommend switching as much as you can over to black.
Hope this helps,