Ok well first like what has been said forget milling steel. 1: The machine has to be Sooooooo much stronger to do it, 2: The rails you have bought if round type are not upto it, 3: The spindle speeds for cutting steel need to be so much lower than even aluminium you won't find a spindle at sensible money that can do it.

Has for the rest then that's very easily do-able with the right design. Regards Aluminium profile then it's no stroger than steel and will cost far more than you probably realise because it doesn't end at just buying the profile, all the other hidden things like connectors and T-nuts can a nd do cost nearly has much.
It's also no easier to build with than steel and all that applies regards setting rails on same plane etc still applies and is no easier with profile.

Steel is a fraction of the price and again if used correctly with right frame design then easy to get very accurate with not too much effort. Apply a few tricks which you'll find by searching the forum and you'll build a very accurate and strong machine for much less than you will a profile machine.

You'll see many references to whats needed for a good all round machine from both Jonathan and me who often use steel. Just search the forum.!

If you get stuck or are unsure drop me a PM and I'll help if I can.