might be buying a 95 caprice cop car for some parts or driving.
Does anyone know what i should look for,which ones where the good years?

Go to 7-11 and buy Cars Buyers Market newspaper. There are tons of dealers selling old police cars. I bought one years ago in Brooklyn right off the BQE.

94-96 were the only years of the LT1 and 4-wheel discs.

91 had TBI 305s only, and 92-93 had a TBI 350, which is still not a very powerful engine.

94-96 MPFI LT1 350s are a much healthier engine, but a little more maintenance. Most came with 3.08 Posi rear w/ discs, but not all. Some even had the small 4.3L V8 which looks exaclty like an LT1, so pay attention to what you're getting and don't just assume it has an LT1.

Check the tranny fluid via the dipstick and make sure it's not burnt. Also, you'll probably have alot of wiring to remove if it was an actual cop car. My cousin bought a 94 9C1 car that was formerly a cop car. It took me all day to remove all the extra wiring harnesses that they simply cut before the car got sold. Also see if it had a tuneup recently as they can be very expensive when it's due for one. So far on his car I've had to put another transmission on it, put a new ECM in it, and put exhaust on it.