If buying from a dealer you're mitigating some of the risks with interstate purchase - there will be statutory warranties in place for the purchase and the dealer will need to comply with the same fair trading laws that every other business complies with, so if the vehicle is substantially other than advertised you will have some grounds to seek appropriate recourse.
I doubt the dealer will let the vehicle off their lot without payment in full but again the risk of this is mitigated by the fact that they're a licensed dealer, so there's a lot more at stake for them if they decide to take your money and run compared with a private sale.
Although you mention the vehicle is unregistered, make sure that it does come with an RWC (or the equivalent in SA) as a guarantee that the vehicle is in a registrable state, and that you're not knowingly buying a vehicle with sufficient defects that it wouldn't be eligible to be registered in SA. (the above is obviously irrelevant if the vehicle is unregistered because it's a new or demonstrator vehicle previously unregistered).
All that said and done, consider whether the risk (however slim) is greater than the small amount of money it would cost to fly down to Adelaide and inspect the car in person and make the payment. By all means do all the due diligence first (pre-purchase inspection etc) and if everything checks out, what's a couple of hundred dollars of airfares in what I'm assuming is a fairly expensive vehicle purchase?
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