Why would your laptop wade through all your pictures etc when booting up?
As long as you defrag it periodically, and turn off that silly indexing feature, you should have no problems.
Is the internal drive close to full, or is there plenty of space?
I don't think there's much to choose these days between the various brands of hard drives - just make sure it's USB2.0 if your laptop is. If it's a "big" one, have one with its own power supply, rather than trying to lunch off the laptop.