You should do a search for NOAA routing charts
for the Pacific Ocean
. They hold a wealth of historical data that will provide a way better "picture" than Nullschool can. Download the months you intend to be out there and see what they are likely to present you, bearing in mind that all these predictions are theoretical.
In the event that you are serious, do yourself a favour and buy the standard version of Predictwind which gets the theory a little more accurate (but it's still just theory).
From SD you will drop quite steeply south to make the Marquesas
and the first 1200 miles will take you through the ITCZ where you can expect gales, calms and squalls in no particular order and that nobody can predict.
Once you approach the Marquesas, depending on when you're sailing you'll hit the SE trades which will take you all the way to the SW Pacific (Tonga, Fiji
, Samoa). This is gold-plated sailing. From there you'll again drop quite steeply south to NZ. As discussed in another thread, the weather
from here is another crap-shoot where predictions are not that great and weather
windows are pretty small.
Another good resource that is often available for easy money
in NZ used-book stores is Jimmy Cornell's World Cruising Routes. I bought an update to mine the other day for 10 bucks! Here you'll find short, concise info that will guide you to a hopefully decent decision.