NZFF

And if I get NZSI, should I get Milford Sound and Queenstown too?

Milford sound maybe ok ..I use Mikes Milford .. Queenstown may cause you problems , has runway bleeds and lag time before textures show .. not a polished product.

Definitely. get them all. Queenstown does have a few issues as Ian mentioned but for me they aren't very significant and, in my opinion, the extra detail around the airport and surrounding area is certainly worth it.Queenstown has become one of my significant departure and/or arrival airports since getting the Orbx airport scenery.