Thanks all (sorry to drop off from the thread for a few days).

For the record, went with Apex who was a bit more expensive than NZRAC, but a lot less than the majors (Avis etc), and was more forthcoming with info about the age of the car (chose middle-price level of a 2010 car rather than cheaper 2004-2007).

Unfortunately we arrive in Christchurch at 10pm so have to use after-hours car pickup, therefore good ideas like making a spare, cancellable reservation elsewhere don't help in our case (and somewhat to my surprise needed to pre-pay, even if the Apex reservation is cancellable!). So we'll just take our chances.

Notice a lot of responders in this thread paid extra for zero excess. In our case, this is covered by our credit card, so skipped that (seemed full of exclusions, anyway).

Thanks for all the help, will try to remember to report back after the rental - but that won't be until end January!