Except if you have a graphics card that is producing a working framebuffer you don't need to add the addtional serial tty configuration. You can then simply run "apt-get install xfce4" you will then have XFCE installed, lightdm should get installed along side the xfce4 package. Now the real problem is if your graphics card will work well enough with Xorg. I have a couple of old Sun graphics cards but didn't really get a good experiance with any of them.
Currently I am going off the beaten path again as I am trying to get an old GeForce I have (NVIDIA Corporation NV34 [GeForce FX 5200] (rev a1)) working on this system again. At one point I had my SunBlade 2500 running at 1080p and able to play Quake 3 with this graphics card. I have gotten the nouveau to load the basic modesetting terminal, which is pretty good, but am now running into CACH_ERRORs when I try to force Xorg to run which is locking up CPU0 at 100%. So still working out those problems. (Maybe I should try Wayland instead?)
-Kieron GIllespie