When buying a used police car, consider that it has probably been used to transport prisoners. Has it been thoroughly disinfected? Prisoner transport sometimes involve biohazards-blood, urine, feces, vomit, and  contagious diseases, such as HIV, AIDS, and TB, to mention a few. Do you plan to put your kids in the back seat where those drunks and drug users were sitting? There's more to consider than just mechanical reliability. Be sure the car has been thoroughly cleaned.