The largest issue you will have is that the idea of Germanic people get's very complicated and entangled when you start looking at scholarship which employs La Tene or Hallstatt definitions instead of Celtic.

The idea of a unified Celtic people and a separate Germanic people is the byproducts of nationalist history in the later British Empire. Scholars attempted to use ancient history to create unity in Western Europe by showing a common ancestral root. The same movement ,which caused German Shepherds to be renamed Alsations and the royal family to rename from Saxe-Coburg-Gotha to Windsor, also led to this attempt at creating a separate historical ancestor for the British and their allies than the Germans and theirs.

For further reading on the original theory have a look at the writings of French historian Marie Henri d’Arbois de Jubainville. He is one of the earliest architects of the theory of the "Western Celtic Empire" (my words not his). However for a better understanding of the culture and history of the region in the iron age check out either s. James "Celts: Ancient People or Modern Invention" or J. Collis "The Celts: Origins, Myths and Inventions".

A better understanding of the regional history will really help answer your question, as it will build a picture not of two divergent cultures but more so of a gradient of culture spreading across Europe.

As a quick side note, I'm not denying that there were other people's and cultures in Germany at the time. Largely migrations from the North rather than the South West but that the notion of Germanic & Celtic is outdated and understanding current theories on the subject will vastly assist your research.