Some of the parts on older guns were hard chromed to give them a silver colored match to the rest of the gun (stainless frame and slide). S&W, due to EPA concerns, no longer hard chromes those parts, so you get hammers, triggers, etc now that are blue instead of silver.
The Novak sights are excellent, and can be easily replaced with night sights for a reasonable cost. The police trade-in guns are an excellent value; most have been fired little, and just have cosmetic wear. Since the pistol is all stainless, it can be bead blasted to refinish it easily.
There are a lot of hi-cap magazines available, because these pistols were very popular with law enforcement. KY Imports, I agree, is a good place to shop for used pistols and mags.
I have several 5906's in various forms, and they are my favorite 9mm. For very little cost, you can get an action job done to improve the smoothness of the trigger. Usually, a $25 re-crown of the barrel will turn a so-so shooter into a very accurate pistol.
I believe the 5906 is about the best value in a 9mm pistol. They are very underrated by folks that haven't shot them, but are excellent values, and a good, solid pistol.