Ryan said it well, the minimum required to complete the task. Being a ham and an EMA volunteer, the radios, scanners, and switchbox I have mounted in the truck are basically a neccessity. Laptop, GPS, data connection aren't neccessarily required for chasing, but as a newbie I could use all the help I can get finding the right storm, and keeping safe at the same time. Video cam to document everything.
This will be the first year that I've used all of the above at the same time, so we'll see how it goes. I think it might become a burden if everything is on at once, only because I chase solo and that's just too much to absorb sometimes!