JohnR
I fly through Germany a couple of times a year en route to Hungary. I have to say I have always found the various Flight Information Services absolutely brilliant as well as ATC generally.
It may be worth purchasing the Bottlang trip guide for Germany from Transair. lots of useful info in there. (although this only has VFR plates)
Also, the German Aeronautical Information Service can be found at:
www.dfs-ais.de/pilotservice/index.html I have stopped at six or so airfields (Bottlang will tell you which have Customs as will the AIS site) and never had any problems. There was a report of one particular airfield that is over enthusiastic on document checking and this is well known to the German GA community. I can't remember the name but I'm reasonable sure it's not the destination you have in mind.
Have fun !