First, thanks to everyone for all the great advice and insights you've shared so far. Our trip to NZ is just a few months away and I'm realizing I need to get the car rental thing figured out.
The plan is to pick up the car in Picton (preferably at the ferry) at the end of May and return it in Dunedin, 12 days later. That leaves Apex out as an option, but Ace, Jucy, Avis, Thrify, Hertz will work (and perhaps others). Ace and some of the others have "super saver" as a car category, for their "well traveled cars." They are considerably cheaper -- I"m less concerned with looks (I drive a 2002 Cavalier that has 150 K miles) than I am with having a car that will handle well and have sufficient power to get us over the hills and through the woods from one end of the island to the other.
I'm also a bit confused about all the waivers, and extra insurance, and could really use some advice.