You can often find these cars cheap at auctions, and online car traders.
Yes they are cool vehicles, but almost always all equipment is removed prior to sale, such as power chip, partition wall radios gun racks and all other police items.
Lucky for you the spotlight is most often left on the vehicle, and obviously the headlights will stay intact! You could use these lights in your garage as well as in a quarry, or on a back road, or if you parked your car in the kitchen you could use the lights there as well. However you battery in your car will run dead if you leave it on for extended periods of time.
Also the headlights and spotlight are 12volt so you can not plug them into your wall socket (110volt) in your house.
As far as people riding in the back, sure they can, it will not be very comfortable, it will almost be as if they have been arrested, also I would make sure to put door handles back into the door, in case there is ever an emergency.
I would wash the seats down with bleach if they are made of leather, that is what Ambulances usually do to kill bacteria left behind from blood or other bodily fluids!