Don't bother looking at an old 'vomit comet' as they're popularly known. Because of the shoddy wiring for equipment(police departments aren't known for their stellar wiring abilities) and various other things, a used HPP crown vic would be much more suiting for most people. here's another list of differences:
http://www.85merk.com/members/liquid/p7x.html
http://www.85merk.com/members/liquid/p7x.html