Having put many many miles on Police bikes in training and work . I have noticed that the maintance on a bike varies from Dept. to Dept. Some have their own motorpool mechanics, and some do great work and some don't.
I've seen them use anti sieze and an impact wrench on loose compensator nuts. "??" The Depts. that lease " or have a special deal made to them by some HD dealers for say $1.00 per year or at least have their bikes maintained at an HD Dealer are usually good bikes to buy.
The bikes to shy away from are high milage one's because they are usually are used in training a lot. That means lots of drops and lots of riding the clutch and rear brake for slow speed manuvering skills.
The most problems with a service bike is they might need a clutch and rotors and or pads.
But all in all they are good bikes to buy. Just watch for electrical issues if they have been civilianized by a back yard mechanic. Sometimes they can cut and chop wires. You won't find a more comfortable seat than the solo air ride saddle in my opinion! Enjoy.