I've bought and sold cop cars for 20 plus years. People buy them for all reasons. You can find a large assortment on ebay from $2000 to $10,000, most with less than 100,000 miles. For the most part:
1) They are maintained excellent, both mechanical and body.
2) They start off with heavy duty suspension, alternator, tires, cooling system, etc.
3) Replacement parts are just as heavy duty if not more.
4) They strip 99% of the police stuff including: Siren, light bar, K9 cage, baton or rifle mount, and the police emblems. If you really wanted the K9 cage, you can purchase that separately.
Unlike TV, most police cars are not continually driven 140 mph in high speed pursuits. In reality, most police cars sit just ideling.
It's a felony to impersonate the police in a used police car.