Proto Germanic

I recently read that approximately 30% of the Proto Germanic vocabulary
was of non IE origin and that much of this non IE vocabulary is
retained in modern Germanic languages. Does this indicate that the
original Germanic tribes did not speak a IE language and that perhaps
they were conquered by or mixed with IE speakers, eventually adopting
their language whilst retaining a large part of their non IE
vocabulary. Alternatively was the non IE element due to borrowings from
non IE neighbours.
Finally, would anyone like to speculate as to the original homeland of
the Germanic tribes. My own guess is that they inhabited the Baltic Sea
area or southern Sweden.
Enough questions for now!
Thank you in advance.
Regards David.