Hi, the basic design seems pretty standard fare, the things that stand out to me are the gantry, it's very tall, but isn't braced in any way, so you'll find you'll get quite a bit flex there. If you can add some right angle bracing that attaches along the length as well as to supporting the horizontal panels at the top and the bottom, you'll greatly stiffen it.
Almost any machine can cut aluminium, with varying degrees that is. Part of the biggest problem faced with cutting aluminium is rigidity, and mass. Machines made out of thin section material, will resonate and vibrate a lot. This directly translates to noise from reasonably loud to screaming banshee ear piercing sounds, as well as very adversely affecting the cutting quality, which in turn translates to poor finishes, and lots of worn/broken cutters, and stress on the machine.
The more mass you can put into the base and frame to add not only weight, but really stiffen and support it, and the stiffer and tighter you can make the gantry, the better it will be for not only cutting, but also the noise produced. Some people have used various materials such as epoxy, concrete, etc, to help with these problems
Using 5mm steel I think you'll find that every single point on the machine that has any form of tolerance being even the slightest out, will result in the problems I highlighted earlier. Another god trick is for every part of the machine that is metal to metal contact, use a strip of the really thin double sided tape, it compresses down to nothing so won't affect anything, but it can greatly help with resonating and vibration noise.
cheers, Ian