Dunnersfella: The Onkyo is HDMI 1.4a and play all the lossless formats, so you won't need an extra optical cable.
The specs do state that it's 'Continuous 6ohm rated', but not 'Certified 4ohm performance'.
Your speakers are 3 or 4 ohms...
And interestingly enough, JB's website stated the amp is 100W, but the US Onkyo site says it's 65W at 8ohms, or 90W at 6ohms.
Either the NZ amp is more powerful, or someone's not telling the truth? Rounding up?
Again, I really wouldn't run your current speakers - and as most amplifier companies exaggerate / gloss over their specs... yeah. Don't do it.
Dunnersfella: Well, why do you want a home theatre system?
As a feature rich toy to play with?
Or something to make your content sounds as good as it possibly can?
Take your pick, because at your budget - I suspect you'll only get one, not both.