@Mazzy , welcome to the forum
Getting to a start point.
As Singapore fly out of Christcurch I had a quicj look at their time table for a departure in April 2019 and return in June, by way of example. My first choice said nearly NZD 3 000 return. But using the "five day" feature I could see a number just under $ 2 000. Of course if you go to Auckland you will have more selection. Beware, some airlines (Thai, for example) do not include airport and other levies.
Why Bareclona?
It is an interesting place in its own right (the fascinating church Sagarada Familia - Holy Family - by the eclectic architect Anton Gaudi, for example - as well as two of his houses).
And from Barcelona you can take a train to Logrono or Pamplona and a bus from there to your start - either Roncesvalles or Saint-Jean over the border in France.
Preparation - body and mind
You have an excellent model in the Central Rail Trail. with one draw back - it is relatively flat along its length.
I started my preparation in February 2012 and started in France (Le Puy-en-Velay) in April 2016. My first training walk (5 km down hill) was a personal disaster. But after a while I had lengthened my training walks to between 20 and 40 km. And I had some hills to go up and down as well.
For the past year my regime consists of:
1) achieving 15 km before stopping for breakfast
2) achieving an elevation gain of 600 metre before stopping ...
The purpose for me is to get mind and body fit for the day after day pattern of:
arrive at your accomodation, wash clothes if required, buy some lunch food, have a meal, consider the next days journey, chat to other walkers, sleep, rise (early), walk, have breakfast, walk, have lunch, walk, arrive, repeat.
The rise early part is to get most of your journey done before the heat of the day becomes unbearable. It is not, as some categorise it, a race for beds.
preparation - gear
I wont develop this except to say it is important. You level of fitness will tell you how much you can carry. Suffice to say walking in Spain is quite different to tramping in, say Mount Aspiring National Park (or my local Tararuas). Walking shoes (not boots), no tent, just lunch for the day rather all meals for a week, and so on.
That's enough from me to start with.
Do you have any specific questions at this time?
Marianne, kia kaha (take care, be strong, get going eventually)