Hello!
The first question would be: VFR or IFR? If you fly IFR, you don't need to worry much. All you really need is a german Jeppesen kit and the Jeppesen low-level enroute charts for the areas you want to visit. Most neighbouring countries have their AIPs online and you can download the IFR charts of your destination and alternate airfields for free. Germany is one of the few countries remaining without this service! IFR flightplans to foreign countries are no different from domestic ones, so nothing to worry about here.
Going VFR is slightly more difficult, because some countries are somewhat restrictive regarding VFR flying (e.g. Italy) or have very strange national peculiarities (like for example "overhead joins" in the UK). You really need to read the relevant sections of their AIP or talk to someone who has been there recently. The best thing is to get the VFR Jeppesen manuals (ex Bottlang) for the countries you want to visit. All Jeppesen approach plates look the same, but VFR approach plates in national AIPs look very different between countries and can lead to a lot of confusion. The introductory part of the Jeppesen Manuals contains all the various national rules too, sparing you the effort of browsing through all the national AIPs. The price of the manual is not cheap, but fines for busting airpaces and violating rules and laws are not cheap either...
Happy landings,
max