PhD student looking for a flat in Cologne (650 euros max)

So briefly, my name is Florian, I am French and I am 24. I am doing my PhD at the Institute of Sport and Neurosciences in Cologne so I am looking for a flat for that period ( 3 years ).
I am looking for an appartment spacious enough (35 m²) furnished (or if not, at the least with the kitchen furniture). My budget is limited to 650 euros all bills included. I have a monthly salary and guarantors.

A little description about me:
Along my studies, I had the chance to travel all around the world (I lived 5 months in New Zealand, 6 months in California, 2 months in Australia) where I met a lot of great and inspiring people. Sharing new cultures and ideas is really something that I appreciate.
After my work at the University of California Berkeley, where I was working as leading investigator, I came in Cologne to do my PhD in Sport and Neurosciences. I arrived 3 weeks ago so that is why, for the moment, I do not take the risk to speak in German (but I will do it very soon).
I am really sportive (track and fields, pole vault, surf, american football...) and I take care of my health hygien. I have also a lot of different hobbies like photos, drawing, guitar playing, litterature...