I lurked until I saw this topic, and I'd figured I'd chime in.
First of all, if your looking for a Panther platform car why not look at a Grand Marquis? Same car, different badging. A mid 90's to early 00's shouldn't be much more than a retired P71 and should be in much better shape.
FWIW, all civi model Crown Vics were discontinued in 2008 and they went to fleet sales only. Some post '08 LX's were sold to dealerships but they command a premium.
But if your set on a P71, the ones that are generally in best shape are detective or admin cars, being the least abused for the most part. Alas, most people that have them know that and want buku bucks for them or if they don't know what they have get bought up quick by people who do.
Agencies that have "take-home" cars is another route. Departments that have their officers or deputies that have an car assigned to them that is taken home with them after each shift. Usually less pooling of the cars and less idle time on the engine, although being a PD car it's going to have a lot of extra idle time and abuse as is.
If the car came from a poorer department usually that means the longer the car was in service and the more beat up it will be by the time it hits the auction block.
City and State(No idea about Canadian) agencies usually "pool" the cars between officers so often times the car is running 24/7. They might be sold off at 60-80K miles but those numbers conceal the thousands apon thousands of hours of idle time on the car.
The Ford Panther Platform is very robust, and a well mantained car should last a long time. It's quite common for Taxi fleets to buy used up P71's and run them for 300,000 miles. On the other hand, it's not uncommon either for people to end up with used up clunkers.
The biggest point on buying these cars is that you need to know what your looking for.
As always YMMV.