Looks like a nice car. I think 20K is a premium price though. That said, if you reckon 10% loading in WA that takes it to 18K east coast. Mm, still a little pricey for a 200Km car. Not too worried about the engine (V8), but lets say a trans rebuild (good one) will be min 4k? Just looking at possible worst case scenario... it could be absolutely fine for many more Kms.
First point of call - a good quality inspection. Pay for it, its worth it. The boring things like struts, mounts etc etc all add up. If this checks out OK and they find nothing more than a few minor things then i'd consider offering 17K tops but preferably 15-16k.
Bottom line. Do you want it? If you really, really do then just get an inspection, and make your offer.