The keyword is really great, but I have no clue of values of german keyword domains in other country extensions.
German keyword + .com/.net/.org can be valuable, but most people in Germany are not aware of .co.uk as far as I can tell.
Buy.de would be much stronger as English is a wider language than German, and a large number of Germans speak English. The reverse however is not true.
i have tried german keywords in .co.uk extension before but no matter how good the keyword was the value was close to zero. if you do german keywords only focus on .com and .net