Well here goes... Aluminium frame router design

[JAZZCNC]

Little better but still not what I had in mind if your seriously wanting to cut aluminium. Check out the forum for machines with base frames with sides raised and rails sat on top. This design is more router biased and it will cut aluminium no problem but not with the same abilty as machine aimed directly to do this.

Also that Z axis design is bad news. Again check out threads and look at Z axis designs that put the rails on the front plate and bearings on rear. Your design is a constant length lever that won't even reach the bed surface and will give the same amount of flex whether it's at the bottom of top of it's travel. With the other design the lever length is variable dependent on material thinkness or distance from tool.

MDF for the bed isn't ideal and I certainly wouldn't waste money putting aluminium on it. MDF changes shape from day to day so if your wanting accurecy then you'll need to surface it often. Personaly I'd try to find the most stable material you can afford for the machine base, Good quality ply will work then skim surface it flat and seal or put the aluminium on that. Then use MDF as a spoil board that you don't mind cutting away or into.