Just check everything as you would with any car ie: tyres, brakes, fluids, leaks, electrics, accident damage, rust, gearbox (hot and cold), clutch (if it has one), radiator for coolant and whilst running, exhaust, unusual noises from the motor, body and suspension whilst driving etc. If it's an auto make sure the gears change up easily and it kicksdown when it should. Series I VNs are very harsh on the change from 1st to 2nd. As per a previous post check for any leak from between the gearbox and enginge.
All the V6 Commodores should respond to the throttle immediately so if it doesn't could be just a tune required but you would think the owner would have done that prior to selling. So beware of the old " oh I was gonna get it serviced in a coupla weeks". Sometimes it is true but many times it is not. Ask them to get it serviced and you'll have another look when it's done.
Injectors are noisy so unless they are really loud it's normal. Coil packs, Crank Angle sensors, DFI module can and do fail occasionally but if the car is running, they haven't failed yet.
Best tips are to get a competent mechanic to check it over first (unless you are one yourself) and to test drive more than one car so you can pick any differences or confirm how they should drive.
If you have any reservations about a car, just walk away. There are plenty around.
