Not that I would ever discourage anyone from buying new tools ;-), but the minimalist answer is a screwdriver, a spoke wrench, and Sheldon Brown's excellent instructions. A truing stand is very convenient but not essential. If you know you are going to do a lot of wheelbuilding in the future, then by all means get all the hardware, but if you just want to see if it's for you, it's really not that much harder to true wheels on the bike using brake pads as your reference points and fliping the wheel to check of dishing.