Make sure your car is properly road worthy.
All lights work, no jagged parts, tires with proper grip, insurance, tax, etc.. Whatever the rules are in your state.
Police probably want a chat to warn you about driving safe and to stop joyriding. If they wanted to charge you for a crime they probably would have turned up at your doorstep.
But you don't want to be pulled over for a chat only to be done over by a broken brake-light.
One issue is that my car is not registered and is wearing plates that belong to my Buick, which is jointly owned by my dad. But I got an extension on the 30 day registration period for another 30 days, so I have until....uh...next Sunday....and the license office is closed then. Better get that sorted out.
http://www.gmfleet.com/police/impala-police-car.html
this one
Those are being replaced too. That generation of Impala is no longer built anymore. The Taurus is replacing the Impalas in my county.
Its a decommissioned Crown Vic, probably sold at a police auction. He bought it from some old dude. It's completely legal, and you need to learn your stuff.
This. It was sold at the Mount Prospect, IL auction (the same one that the Blues Brothers bought their car from according to the movie), then I got it off an old guy.