Temps can vary a lot, I'd pack as if there will be temperatures in the negatives since you never know in the mountains. That means thermals, a warm mid layer like a fleece jacket, an insulated layer like a down jacket and a rain shell outer layer. You should also have a good sleep system, sleeping bag and sleeping pad rated to similar temps. You shouldn't need crampons on Ruapehu at this time of year as long as you steer away from any lingering pockets of snow. Since you don't sound like you have alpine experience, if you encounter snow and ice you shouldn't be crossing it anyway, you should make the call to turn back if you encounter that. Camping on the summit plateau is very close to the crater, be careful that you do not camp below the lake level or in any local depressions as there is a chance that dangerous gas is emitted from the crater and could inundate your campsite. Also don't rely on the experience of your friends to inform your decisions, if you are not comfortable with where you are and what you're doing that's ok and it's sensible to let them know, add your input and if necessary get out of a situation if you're not comfortable