After more pondering and looking around I've reached the conclusion that I really want a T-slot table.
The only reasonably priced extrusion in Aus I can find geared towards this is from
Australis Engineering
I could fairly easily bolt that on top of the design above.
Total cost for frame above + T slot would be approx $600
without gantry sides
That got me back to thinking about using 30 series profile which has slots that match the hole spacing on my supported rails.
A frame made entirely out of 60x60 (running in the long axis, 1.2m long beams x 8 with no gaps between joined by ends plates +/- a strap or two underneath) comes to around $500 (again without gantry sides).
The 60x60 option would be pretty much bolt together.
Thoughts?