If the odometer numbers are skewed, it may simply mean that the previous owner (in Oz or Japan) rather than the current owner has tampered with it.
In the adverts, he mentions that there was a previous owner he bought it from > so is there any paperwork?
Does the previous paperwork including auction papers from Japan mention anything about the odometer reading?
One really needs to go by other telltale signs as to whether or not the car is in good condition and has a good service history.
Check it over with a mechanic who knows Skylines > comp test and the rest
...and a panelbeater friend who can check for rust, previous panel removal, damage to wheel well, crossmembers, arms, struts, rails