Thee 5906(and its relatives) is one of the 9mm great guns. The only thing I would replace up front is the recoil spring, and if a mag malfunctions a mag spring. Keep the old recoil spring as a spare. If you have a recent 5906 do not even do that unless the gun indicates it needs it, like failing to strip a new round from the mag etc..
As to the sights, that is a personal preference situation. The Novak sights are as good as they get, and the only reason I would replace them is to get night Novaks. I would shoot the gun with the issue sights, and see how it does. If you are getting good offhand groups that are POA, I would not touch the sights-don't argue with success.