Looks good, but if you plan on milling steel you must design it for lubrication also, and you must make the bed so that the cutting fluid can flow without leaking to the floor. Also, design it from the start with the right spindle in mind. Steel requires low rpm and high torque, MDF, plastic and aluminum can be milled with high rpm as well. So, either make the Z able to take two different spindles or make it somehow universal, but MUCH more rigid and larger than the Kress requires.
Otherwise I like the idea of fixed gantry. It is how I designed my CNC as well, and the reason for that was simple... It is easier to make a more robust machine than the more sexier looking moving gantry allows. I am happily milling aluminum as well as delrin with very good results today. Not tested steel but because I don't use lubricants I can not mill steel.
Another reason why I like fixed gantry is that it is much easier to encapsulate the machine to protect the surroundings from flying debris and dust. I like the way you designed the protection. I chose a simpler solution, and also not covering the top, only the sides. In my opinion protection is necessary in these type of machines. I have had several broken bits and when they break they become dangerous projectiles. Also all the chips are kept inside the machine, so the room where I am using mine is easily kept clean.