Welcome Pippin
You got a really nice tank to start with there!
A CF2700 or a FX5 are good external filter for tank size at 450L. Coral sand plus some rocks is my favorite setup.
Malawi fish that doesnt get too big, generally you are looking at Mbuna cichlids. Electric yellows, demasoni, zebras cichlids, ice blue, some nice peacocks and more.
For your tank size I wouldnt worry too much if you start with young cichlids. You can put in some nice haps as well.
Depends whats available around in NZ and good luck on your new tank setup.
Some website for you. You can always google it
Mbuna cichlids
http://malawicichlids.com/mw07000.htm
Haps cichlids
http://malawicichlids.com/mw05000.htm
Cichlids complete list
http://www.cichlid-forum.com/articles/african_cichlid_genus_gallery.php