There’s nothing redeeming in Te Anau. I stayed at the Milford lodge which is essentially two seconds from Milford Sound. The views down there are gorgeous. I don’t see how you’d do this with a van but I wouldn’t stay anywhere else if you want to visit the Sound.

Christchurch really sucks and is pretty far from everything so I wouldn’t recommend it as a base. I hear the picton ferry can be rough. I recommend flying it’s not much more and much faster

In general NZ is best done with a car. I spent one month in NZ and one month in OZ. Sydney is great, try to visit the blue mountains if you can and do the great ocean rode as a day trip from Melbourne