I'm going with BP Retired on this...I have been driving retired police vehicles for the past 11 years and they have been, by far, the most reliable and durable cars/trucks on the road.
I also got my retired BP SSV from Chicago Motors a couple months ago and could not be happier. The interior looks brand new, engine compartment is clean, and the body is straight. Take your time looking at what they have coming in as their inventory is updated constantly...I think Chicago Motors has the best prices around. These guys are easy to deal with and shipping costs are CHEAP!
Don't be afraid of the used police vehicles...Plus they look badass!