Hi Rik, check out the JustMusic shop in the Kulturbrauerei Prenzlauer Berg (Knaackstraße 97).
There's another one in Berlin Wilmersdorf, which I visited only once, since I tend to like the shop in the Kulturbrauerei better.
For vintage guitars also check out Checkpoint Guitars (next to Checkpoint Charlie), they have some cool stuff, although mostly overpriced.
+1 on Just Music, bought a wireless there, nice folks.