not to be a downer but when buying a former police vehicle ALWAYS assume that it was a full out patrol vehicle. even if it was a "supervisor's" vehicle, it idled constantly and that in a grand scale of things is just the same as putting on miles on the car. We have supervisor vehicles pushing 5 to 10000 idle hours, so you might be getting a low mileage car but the high idle hours would be the equivelant of adding on tens of thousands of miles on the odometer.
I have owned 2 crown victorias and a SSV tahoe in the past, not to mention that i drive the cars at work. You want to pay attention to the details of the vehicle. Look at the vinyl flooring, any extreme wear will be a red flag. Same goes with how much care was put into installing old equipment, scratches throughout the exterior might tell you that hey this thing has some battle scars (highly doubt it was admin at that point). The way equipment was removed might tell you how well it was cared for, if the wires are correctly plugged or sealed and the roof holed are carefully capped then the maintenance department actually gives a damn. door hinge issued are pretty common on patrol tahoes, the door fails to close properly and might be a sign of a lot more use than you are being told.
Supervisor line is used all the freakin time in used police car sales, I would not believe it for a second. Do a full inspection and make sure everything checks out.
Also, running carfax wont show you issued with fleet vehicles usually, you want the car inspected for any damage, quite bit of patrol vehicles come in contact with objects on the road (barriers, buildings, dumpsters, curbs, poles, poles, poles lol, and so forth....) none of these repairs are reported but have potential to cause major driveability issues.
But all that said and you take the right precautions, you can get your hands on a killer deal, just do your careful research and enjoy what could be years of care free ownership