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Common problems with the vz range are timing chain cover oil leaks, the usual castor rod bushes, radiator cap, ecu failure, tailshaft couplings, thermostat housing leaks, and the normal timing chain rattle on cold starts. If I were u, buy an sv6 06, they are the newer engines and gearbox and run like a dream. The 175kw engines are a gutless piece that will only disappoint, at least the sv6 has a bit more go and the shorter ratios in the 5 speed auto make it feel quicker too. Downside of the vz is the throttle lag due to the drive by wire.
a VZ is good choice , I've had two VZ's and am happy with them ,I prefer them to VE,s 15K should get you something decent, try for a one owner with service history and take your time looking cause something good will pop up.
cheers Pete