As it is with buying any used car, there is always a degree of risk involved.
As long as the car has a full service history (Preferably with the intermediate 6 month ones done also), you're going to be a safe as you can get.
If you are buying used from a dealer, there will be at least a 3 month warranty on a car that is under 10 years old, which is reassuring to some degree - Unless you're dealing with a real dodgy place. If you are buying privately, without a doubt, getting a full inspection prior to purchasing is advisable. Obviously if something major is found, you know to walk away. If it's something minor, you may be able to talk the seller down a bit to pay for those repairs... Well worth spending the money on this, even when buying from a dealership. It's always nice to ask hot the mileage was done. You may find it's high because they live 60km's from work & it's mostly highway km's. Never a guarantee they'll answer honestly, but worth a shot
Based on the amount of older Holden (& Ford) V8's getting about, they really do seem to stand the test of time when looked after.