There's a big BMW dealer (bikes and cars) in Aachen not far from Dusseldorf. www.kohl.de well stocked with new and second hand bikes and lots of rider equipment too.
Was standing in the showroom at that address last week and will be again in a few weeks time, it is the only break from the inlaws i have when visiting
Its an okay dealership, the one in Aachen is much better, depends what you are after. Their clothing department is upstairs at the back.
Since I bought my last few bikes there, I can confirm there's a bike dealership there!
The main car dealersgip is on one side of the road, with bikes and used cars located opposite. If you need any contacts there, or any other help (translations?), let me know.
There a several bike clothing accessory shops located close together about 5 minutes walk from the station (turn left out of the main entrance) - Hein Gericke, Louis, Dainese, Polo etc.
The altstadt is quite a nice place to spend an evening or ten eating and drinking I used to go to the Irish bars to watch Leeds playing in the Premiership when I worked in Germany