A Basic or Standard license just makes this process nearly impossible to automate or model. I've programmed this in Python with the use of Advanced license tools. See this discussion on the problem of just buffering a coastline, which is the identical problem you face (buffer coastline only is just another name for buffer external boundary only). There is a python code file that shows the Advanced tools that you would need and snap shots of the problem areas I found in the results shown in post number 8. However, post number 9 shows a snap shot of the more typical good results I got.
I have recently created an ArcObjects add-in that does left and right hand buffers of lines without overlap and that would work with a Basic license. That tool came about as a result of this discussion and I have resolved most of the technical problems since my last post to that thread. At this point, the took works but I have yet created a universal user interface so it is user friendly enough to publish the tool. You would still need a way to extract just the outer boundary of Germany as lines. That can easily be done with Advanced license tools, but not with a Basic license. At a minimum, I would have to write ArcObjects code to replicate the Features to Line tool functionality to make it work with a Basic license. Since that would take several days to do, I would only offer to have you send me your polygons and I would just run the Advanced license tool to get you those boundaries in under 5 minutes.
Unfortunately, really wiggly lines will cause severe problems with the buffering methods I used in the Add-in, but there are alternative buffering options that can avoid most of those problems.
At the same time, your illustration is for all practical purposes impossible to program, since you are using a normal angle to your boundary lines that considers the overall line trend and avoids inlet pockets. You drew lines that are not straight to cut the separations between the German subareas, which is illogical other than to avoid inlet pocket obstacles or to follow another country's boundary. Nothing can really do what you drew other than the human brain or a monster program. For practical and performance reasons geometry can only consider a limited set of points and line segment angles to come up with a normal angle to the wiggly boundaries of Germany.
Also, do you care about the boundaries of the adjoining countries? The buffer you drew could cut in and out of multiple adjoining countries across the lines separating each of your subareas.
Anyway, this need comes up all the time and I am having some real success at tackling it for several scenarios. But I do not believe you can come up with anything using Python and/or ModelBuilder that will do what you want under a Basic or Standard license only. If you really want to do this level of work, a Basic or Standard license is just not adequate and you either need to buy an Advanced licence or partner with someone who has an Advanced license.