For local info on importing a Mustang, Phil Hart on 08 8552 2122 has specialised in Adelaide for many years, and owned quite a few Mustangs, so he would be worth a chat.
Or Matt from Shermatt in the USA on this forum, has a good wrap for sourcing and shipping cars out.
Think carefully about whether you want to keep a car LHD, or go RHD, as that's probably the biggest decision to make after you decide on a budget, and the year you like. And before you buy, check rego requirements with your authorities so you don't end up with a problem there.