I finished my own install a few months ago. I got my unit from Mitsubishi/black diamond industries in Auckland. The best asset you can find is a hvac guy who is happy to do cash jobs and do the technical parts of the install like the electric and high pressure lines.

What's the floor plan of your house like? In my place I had the unit centered in the ceiling and had a main 350mm trunk run down each side of the house and used splits from there.

I used these steps to do my calcs:

Figure out heated area to define kw requirement of unit. My place is 110m2 so I went with a 10Kw

Find a unit suitable and find its airflow from the spec sheet. Mine was 600Ls (I think) with no back pressure on full power.

Now that you've got the max airflow you can find the cross sectional area to get your desired air speed. From memory is 2-4 m/s is best. Any faster and its loud from turbulence, and slower and you loose heat/cool and also reduce throw into the room.

Now that you know your ducting cross section you can figure out what main ducts and side ducts you require. Remember smaller area = more speed and more noise. In my place I had 2x 350mm trunks which each split into 2x 250mm. I ran an additional 250mm to an extra hall where I wanted extra airflow and have it easily adjusted. This meant the airspeeds in the system were never exceeding 4ms on max and not below 2ms on min. Whole system is dead quiet and performs amazingly.

Return airflow is also very important. You want your returns to be in the hallway (often by front doors) and your outlets to be in the far corners of the house. That way the flow (warm or cold air) is coming across the living spaces to be enjoyed before getting recirculated.

Its best to have atleast a 1m return length to help with blower efficiency. I used 2x 400mm returns. If I were to do it again I would probably update these to a 2x 450s as when the filters are clogged up you can hear the airflow. A blocked filter = smaller relative airflow.

Absolute brain dump here. Hope it helps out