Here are some cars from the TDM pack that could possibly serve as police sedans (Ford Crown Victoria, Holden Commodore SS and Mitsubishi Evolution pictured):
Here is a picture of a police SUV which is also part of the TDM pack:
If people are interested, I can apply the Cavalcade vehicle script to the police SUV model when it is released and make from-scratch scripts for the sedans if they are required.
Just so you are aware, the police vehicles (the Crown Vic, for example) were never released, as he has been waiting for an update to the vehicle script he uses. There's also rumors that he's left Gmod, but they haven't been confirmed. The only released Crown Victoria is a taxi version.
I feel that this idea is a good one, and it should be restricted to gold members ideally.