I won't go as far as to say it's impossible though. But in all the T slot aluminum builds I've looked at, I've never seen anyone pull it off. It would have to use much bigger sections than are typically used.
For the spindle 6.5 kw should be more than enough power, but you would need power available at the lower RPMs that you would cut steel at. The bits you want to use are small in diameter, so perhaps the spindle would be OK, I can't tell you that off hand.
How big are the parts you want to cut? The best idea is to use a plasma cutting machine to cut large sheet in 2D and buy a bridgeport mill and retrofit it to machine smaller parts in 3D.
What you are talking about will cost much more money than a typical router, and will need to be very stiff, and easy to mess up in the design process. But not impossible.
500mm is just shy of 20 inches. So if you want a cutting area length of 30 inches max lets say on your longest axis, you are not describing a CNC router, you are describing a DIY VMC machine. If you can find a good used machine that will accommodate the travels you want, you may be better off, this project will be alot of work.
Hmmm, you never said T Slot. You just said profiles. That could be anything. OK, yes it is possible but you need to know what you're doing and spend alot of time on the design first. And it won't be cheap.