Hi Phil,
First of all you are asking what to use for steel, my reaction is "don't".
The gantry and side rails on the bot are extruded aluminum. I wouldn't try to cut anything stiffer than the tool I am using. To mill steel you really need a machine built for it.
I do quite a bit of metal, but we are talking brass, copper and aluminum.
For those, I use "niagra cutter" non-ferrous bits, both 1/4" and 1/8". I get mine from centerline industrial as I have a shop local. They do online orders as well though. I have not found a need for cooling, though I do use a cutting lubricant called nixstix. The wax (I think) based formula comes in a push-up tube that you can rub on the bit and the work piece.
Chris
Do not follow where the path may lead. Go instead where there is no path and leave a trail.