The reason I didn't mention the Dash series is that I know next to nothing about them. Other than SAS had major (electronic) troubles with the -400 when they introduced them after the 2000 left.
Some of the airfields are 1300m in length.
My thoughts, the 340B or B+ due to $$$. Then re-look in 3-4 years.
The -42 would allow seamless training for two fleets (if MC could accept that!) and the various benefits that fleet (almost) commonality would allow.
Or heck, the combination (light touch paper and stand well clear!!) of the two companies.
Who knows. Just as long as they keep passing into Air NZ!!