I have two suggestions right off the bat.

As a Mercedes Benz fan, I would definately go to one of the Mercedes Benz Niederlassung ("Super Store") and cruise through the new car showroom and get a pile of brochures and then into the parts department and accessories botique, and maybe pick up one of those hard to find euro parts, and a couple of neat souvenirs (little model car, keychain, drinking mug, ball cap, etc)

Hamburg Niederlassung:
http://www.hamburg.mercedes-benz.de/content/germany/retailer-2/niederlassung_hamburg/de/home/passenger_cars/home/about_us/location.html

Dusseldorf Niederlassung:
http://www.rhein-ruhr.mercedes-benz.de/content/germany/retailer-4/niederlassung_rhein-ruhr/de/home/virtuelle_bu/home/about_us/rep_niederlassungen_rhein_ruhr/rep_niederlassung_mercedes_duesseldorf.html


Above all, though, ..
I'd go see C F Mirbach, it is like a museum of classic cars for sale, they have a store right in Hamburg.

They have some of the rarest and most beautiful collector historic and special interest cars, Maseratis, Gullwing 300SLs, the best of the best.

http://www.mirbach.de/main.asp?lng=en

You should have no problems as lots of people speak good English.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)