I have considerable experience in oversighting the servicing of Highway Patrol cars in NSW. Because of OHS issues, every available safety option is loaded into the cars, so, in the case of VZ's, you got all available airbags, anti-lock brakes, traction control and any other items which helped the cars handle high speeds and safeguard the police officers' safety. Sure, I realise that most of those things were standard in the SS, but some states don't specify the SS for HWP work, despite Holden's advice not to upgrade Exec's for high speed work.
As far as maintenance goes, the cars are serviced at 10k intervals, not the factory 15k. Tyres are replaced well before they become illegal and brakes etc are kept right up to par. Whilst the vehicles can lead a hard life, it is worth trying to find out where they were assigned during their police lives. A car that has worked in Sydney or Newcastle will generally have operated at lower speeds and run on better roads. Country cars will have seen higher speeds on poorer quality roads. Vehicles from out west may have operated at warp speed over dirt or rough bitumen. Thus, suspensions, steering and brakes will have worked a lot harder on those cars from remote country locations, but their engines will have been operating in open road conditions rather than congested traffic.
You have to look beyond the interior and external paintwork etc, and check underneath for stone damage, dented floor pans, dinged lower sill panels etc., but they can still be good value.
Regards
pj