How hard is German?

It all depends.  If you already speak another language, learning another is far simpler.  I took German in high school and college and learned more about the English language than I did from English classes.

English is based on German (and French) for the most part.  In fact many nouns have similar names and history.  English is only a few hundred years old and if you were to go back 700+ years it would grammatically be not to far from German.

Practically everyone speaks English in Germany -especially young people.  You could easily learn some basic German and I would recommend getting VTUNE that gets you access to television shows streamed over the web so you could start to put your new talents to work before you go to Germany.

Also, worth noting, is that many courses are taught in English too in many of the universities overseas. The challenge for you will be attending a class that has difficult concepts (regardless of language) and having enough developed language skills to understand those concepts and that could take years of language study to truly understand the course curriculum.

I agree with what bwalle says.  All I can add is that I found it easier to acquire a satisfying reading and listening level than an equivalent writing and speaking level.  Understanding German content produced by others is one thing -- producing that content is another.  So you may be able to read the academic literature before you can write it.