Kai,
All sat navs will have a log of: Current speed, maximum speed, average speed, time driving etc etc - you just have to find it within the interface of the system. As I said before I prefer the Garmin but you may find other people will prefer TomToms or Navmans. At the end of the day they will all do what you want, it'll just be down to personal preference.
Does you car have a double DIN area for your stereo? If it does then check out the Kenwood DNX7220. I have it it my car and its a pretty good set up.
http://www.kenwood-electronics.co.uk...media/DNX7220/