Where abouts will you be coming to?
Check the Australian import regulations carefully. There is a '1 year' clause, though it may only apply to Australian citizens, and be different for you. It basically says, you must own the car for at least 1 year (and presumably demonstrate you lived overseas with the car), before you can export/import it.
1) perhaps the 2.4JTD makes the most sense, for its economy and hauling torque; Q4 would be a bonus, but not as important over here when there's no snow/ice in most areas; otherwise, a V6!
2) Aus-spec cars tend to be 'more loaded' with standard features/trim, compared with overseas; in the UK, you have a big options list to choose from, but not over here; in particular Ti cars have virtually everything on offer as standard (only front parking sensors is unavailable here)
3) parts are no issue; we have the 1.9/2.4JTD, 2.2JTS, 1750TBi, and 3.2JTS V6; our V6 is Q4; we have not had the 2.4JTD Q4, but presumably the Q4 parts are interchangeable with the V6 (eg: centre propeller shaft); however, parts are cheaper from overseas, and most enthusiasts will purchase from places like the UK, and have them fitted here (eg: brake discs, cambelts, etc)