dont even try and install any gui stuff. (networking should get setup even with the most minimal setup. Just do a very minimal install test networking (ping) then do a dist upgrade to testing and then installl X stuff.
One thing, but if your networking was working during the install why did you install firmware-realtek. I would try removing that. Linux keeps networking stuff in so many different places now its difficult to know where the setup files are kept.
I never venture out of stable nowadays so my advice may be outdated.
HTH
dan