As scenery, it is reasonable - even desirable - to have a mix of cars from the twenty years or so prior to the nominal time frame of the layout. The material that the models are made from is largely unimportant, and if the steering wheel is on the wrong side, face it the other way...
As a load for a motorail wagon there is still a need for a variety of cars, though the bias should usually be toward cars that are closer to the layout's nominal time frame. Weight becomes important, here. Diecast models over the bogies will give the wagon stability, but, a full load of diecast cars will yield a very heavy and unstable wagon indeed.
As loads for goods wagons, the cars required are very specific. The cars are fresh off the assembly lines, and are all from the same manufacturer, probably all the same model - but there is still variety: there are sedans, wagons and utes, in a variety of colours. There may be a wagon load of Valiants mixed in with the Holdens, but the odds are that they were built in the same week, so will look very unusual if they are not the same year's model. Now, weight is critical. Eight or ten car wagons on a single service was not unusual, and, while recent model locomotives are relatively overpowered, there is good reason for wagon weight standards (who's ever you may follow).
Following on from Auscision's announcement of RTR car carriers to come, it is time to consider the availability of suitable loads - and from where I stand, there is not a lot for consideration. So what is, or was available that can be used to load car carriers on Australian layouts?