Mike,
Things to check
CAR number should be stamped on firewall below ID plate. Just the number (not the complete code YKG etc).
Original ID plate should be pop riveted in place.
The BODY number and PRODUCTION code should be stamped on the radiator shroud. The BODY number should be approx 500 less than the CAR number. The production code should be M2 06 for a genuine S shell.
The car number was stamped on the firewall AFTER the car was painted. The numbers on the radiator shroud were stamped BEFORE the car was painted.
As DRMINI says the engine is a MK2 from late 69 early 70. I have a MK2 engine in the 545xx range from a car with a build date of 2/70.
The RED roof is not original, at least not from the factory, as the only colour on the ID plate is the Toga White body colour. If it came from the factory with a red roof it should be on the ID plate. Most cars were two toned from the factory, mostly with a white roof. The cars were painted completely in the body colour and then the roof colour painted over the body colour.
The aircleaner is NOT correct for a MK1. The cutoff corner was only on MK2 cars. (It's to fit LHD cars fitted with plastic tank master cylinders)
It should have metal tank master cylinders and the copper piping has to go. It's unsafe and I believe illegal. Copper will workharden and crack due to vibration. (I think what you see on some UK cars is copper coated steel bundy tubing for rust prevention and is not necessary in OZ.)
The sill trim strip as far as I know is still readily available although I haven't bought any for a long time. It still seems to be in the Karcraft catalogue.
It wouldn't have flairs unless it was originally registered in South Australia.
There were only about 5000 MK1 OZ built cars with the first CAR number being YKG2S2 501.
S engine numbers start at 30000 with a few different prefixes but ALL starting with 9F and all sourced from the UK. OZ built cars should have an engine number prefix of 9F/Sa/Y for MK1 and 9F/XE/Y for MK2. There is some argument about whether the "Sa" actually stamped on the block is with an "a" or an "A". It's allways seems to be shown as an "a" on printed paperwork/ manuals/ parts lists etc.
For $15K its pedigree would have to be unquestioned with matching numbers and fully documented with comprehensive paperwork. Unless you can get it a lot cheaper give it a miss. Depends on how much you want a provable genuine car. Probably best to repair your own known car.
Hope this helps
RonR