your saying you want to mill 5mm deep with a 5mm end mill in steel? Thats not too good of an idea unless its a rougher, and if it is, it doesnt take quite as much.
I dont know how big of a machine your looking to build, but now that I think about it, My machine only has a 450w motor on it, its a seige x2. I havent milled much steel on it though, you need to run it quite a bit slower than how I run it. The way I have my pully ratios wouldnt be good for milling steel.
I used a 3/8" rougher in some steel just surfacing some bar, I took about .05" wich is nothing at a decent speed, seemed to handle it alright.
But, of course, more power is better I do want to get a bigger motor some time or another.