It needs to be integrated into the initial design in some cases. In some designs ducted may not be able to be added very easily due to positioning of A/C unit, make-up air position or even running the ducting plumbing through beams..... So yes, cheaper to do it before construction or before construction progresses to far.

Ducted, 2-zones (bedroom and living). Check out reviews of different systems on productreview.com.au. Look to improve your ceiling insulation, window covers etc to improve its efficiency. We also went a slightly bigger system than we needed, so it never runs at 'full' capacity. Very cheap to run, compared to an under-sized system we had previous that just constantly ran at full pelt.

Wall-mounted AC units have a directional outlet on them, so you'll be able to set it to push air across the room. 7.5 metres is a long way for it to travel though, probably won't go that far. The fans are not that powerful. As for the output size, the rule of thumb for these things is 100W per square metre, so 5kW is about right. With 7kW you should be okay. However your desk is right by a north-facing window so that's where the sun will come in and cook you, unless there's some shading. The suggestion for a fan is a valid one, and will probably make a big difference. I'm not sure is position 2 would really be a lot better since it's directed at the stair void anyway. Thermal comfort is always a tricky one since it's very subjective. Ultimately you'll just have to try it and see whether you feel too hot or cold. But overall I think it won't make too much difference which of the two locations the AC unit is in.

@Ryan Baker - Thanks for reaching out. Your question is difficult to answer without seeing your property. In general, a ducted system can be installed in either the ceiling or underfloor when access is available. Both options provide airflow although, underfloor installations will be slightly more energy efficient when heating as hot air rises and ceiling installations will be slightly more energy efficient when cooling. However, the difference is not significant and would not cause much of a difference in energy costs in most situations. Also, you have to think about access not only for installation but also for future servicing and maintenance. Say for example a duct tears in the years ahead, how challenging will it be to repair if in the ceiling vs underfloor. This may be the reason the installer is suggesting ceiling installation, but again it is challenging to say without seeing ourselves. While Wollongong is slightly out of our regular service area, you could call or use our contact form to see if we are available to assist with your installation. Otherwise, we suggest talking more with your installer about the pros and cons of ceiling vs underfloor installation with regard to your particular property. If you are not happy with their responses, then seek other quotes. Wishing you all the best.