Why Germany? Magenta (UK) do a cheap heterodyne kit that is good value for money; they also have a few ready made heterodyne detectors. Batbox have recently come out with a cheap functional frequency division detector also. Both, as well as some others, are sold by Alana Ecology (<!-- m --><a class="postlink" href="http://www.alanaecology.com/acatalog/Magenta_Bat_Detector_Kit.html" onclick="window.open(this.href);return false;">http://www.alanaecology.com/acatalog/Ma ... r_Kit.html</a><!-- m -->)
I have the Magenta heterodyne kit (which wasn't overly difficult to solder together, I just made the mistake of putting the speaker on the rear face of the box, rather than on the front...), as well as the Batbox Baton frequency division detector.
Most of what we have around our house are pipistrelles and noctules.