You should first probably think long and hard about doing this sort of thing on a laptop. Small screen, limited resources, less raw power, thermal concerns. External drives are added bulk and hassle, plus they're far more sensitive to every little bump and jolt, so tend to die pretty early deaths. These sorts of tasks are really better suited to desktops. Unless you absolutely need the portability of a laptop, consider an iMac or something.

Now, if you insist, I will state that the amount of added heat a HDD will add in the grand scheme of things is insignificant. It's more than zero, but the HDD sits right up against the aluminum case of the laptop, and aluminum is an excellent conductor of heat. That's why if you pick up a can of cold soda, it feels so cold. The aluminum is conducting heat away from your hand into the soda. Aside from just aesthetics, the aluminum case on MBPs helps turn the entire case into a giant heat spreader. You can say whatever you will about Apple, and a lot of negative attributes are well deserved, but their system designers are really VERY good. Of course they aren't really designing laptops with high end video editing in mind. Something you should keep in mind before pulling the trigger on the purchase.