In Debian you have the update-rc.d program for dealing with symlinks of the runlevels. Get the default runlevel to 2 back, than run, as root :
update-rc.d -f xdm remove && update-rc.d xdm start 99 3 4 5 . stop 99 0 1 2 6 .
Then the system will boot with command line. You just have to be able to launch 'init 3' to get a graphical login. You can use sudo for that.