Changing the dk icon to another and editing the info in the text boxes on the right side
The information inside when you create the character? That's inside of Data\enXX\patch-enXX-3.MPQ under GlueXML called GlueStrings.lua. You'll see CLASS_DEATHKNIGHT and CLASS_DEATHKNIGHT_FEMALE. That's where you'll change their information. Not far from that is CLASS_INFO_DEATHKNIGHT0, CLASS_INFO_DEATHKNIGHT1, CLASS_INFO_DEATHKNIGHT2, CLASS_INFO_DEATHKNIGHT3, CLASS_INFO_DEATHKNIGHT4, and CLASS_INFO_DEATHKNIGHT5 with all the ability information they can do/use.
Next you should go to Data\enXX\patch-enXX-3.MPQ under FrameXML and find a Lua file called Constants.lua. Since you changed the name of the class, you'll need to rename everything that is in strings "DEATHKNIGHT" to your class name. This will fix the who list not showing your class when you do this.
About the Icon, I'm not sure where the image is located..