As a owner of several retired police cars over the years, I will say the following:
Early (early 90's)Crown Vic's had only a couple of extra circuits for radios and light bars, they were chipped for extra power and the brakes were heavier duty. Same engine/transmission. Looked identical to the civilian version.
Later versions, made after the civilian versions were stopped, had more power circuits, beefier alternator/battery. Flaws in the engine heads were fixed to stop burning oil, transmission upgraded, chipping required as used high output fuel injectors. Upgraded brakes, oil coolers and transmission coolers were also added in later models.
Maintenance costs remained the same for the most most part between model years.
Idling is the worst thing that you can do to a car, most police cars are never turned off. Police departments started installing hour meters and rated it at 15mph, so oil would be changed at an appropriate time. Ford added a runtime feature for that calc.
Because of the extended idle time, hard acceleration and braking, these were classified as severe duty.
Using an all electric vehicle would be a huge cost savings for a department, even when treated as severe duty. Why? No gasoline costs, electricity is far cheaper; maintenance costs lower, as no oil fluid changes required. Most departments have a contract with a body shop/mechanical shop (or their own in house) for maintenance and repairs -- would specify common parts on hand for quick turn around. In a RFP, for the # of cars there would be a spec for spare parts included, but would be more on the body side instead of belts, water pumps, alternators and batteries (for example).