Kickinbac:

Sounds like you need to find Matt Risinger on YouTubes if you haven’t already. He is the building science nerd and loves a good attic plantroom with MVHR etc.

Also look at external shading options for the rooms you think may overheat.

This guy makes the NZ HVAC contractors I've talked to look like absolute amateurs - though they are paying 4-7% of the house price for HVAC, so a million dollar house would be $40K to $70K. Some points I've learned watching a couple of hours of videos this evening that they do in the US:

• Central dehumidifiers (not really needed here)
• Vibration dampers on the indoor units to such as springs, rubber mounts, etc
• Mostly using rigid ducting as straight as possible which will last 50 - 100 years, with some flexi near the ends to reduce noise which might last 20 years. They also point out flexi duct should be straight, flat, pulled taught, rather than laying around like the one I just had removed
• Central really good quality filters almost HEPA grade inline with the HVAC system to take out dust and pollen rather than the simple flat / sock filters we usually get here

Some videos:

• This one is long and quite thorough
• Expensive system design
• This one is good and shows a spring damper system and rigid ducts instead of flexi, 
• Informative video on diffuser selection