Get the Zex wet kit - you want a wet kit (it adds fuel as well as nitrous
through one inlet). The kit is fairly complete, but for safety get the Zex window switch as well and don't rely upon the Zex "nitrous management unit" (NMU) to keep you from spraying too low. The NMU is supposed to learn when you are at wide open throttle and keep it from spraying, but it will read WOT very low I found out (was getting staged one night and had the system armed, brought the RPM up to 1700 or so and was bumping into the beams when the NMU read me as WOT and fired off).
The window switch lets you set an rpm window for when the NMU will "see" the rpm signal and it won't let the system read too low, plus it will shut off at whatever upper limit you choose, so if you have a stick and miss shift it doesn't rev up too high while the juice is flowing.
Get a pressure gauge for the bottle and a tune for the car as well.
Depending upon where you live you may need a heater. Here in Michigan when I was running a 75 shot of nitrous in the spring and fall, and the cool summer evenings I needed the heater to keep the pressure up. A purge is nice, but not required. It does let you get fresh nitrous up to the inlet though after you do your burnout and while in pre-stage.
Nitrous is a lot of fun, but it is a track only power adder. It costs about $4 a pound for nitrous fill in the 10 lb bottle, and on a 75 shot I could usually get 10 passes before pressure started dropping and performance fell off.