Well done Marcus! The specs look great; looking forward to seeing some high-res images from you! ... and I understand that you will be the first OS owner in Australia - on the leading edge again, I wish I had your courage!!
In reference to your reasons for preferring a truss tube, tube currents and cool-down time are not necessarily an issue for closed tubes provided that they have a good (filtered) air flow system. My "Bunyip" OTA tube is 1625mm long and does not suffer at all from these two issues, having three fans and a large filter to draw the air through. It would, however, be quite nice to have ready access for light cleaning as with a truss tube.