Since you asked this question in the HSM forum AND considering that you will be machining aluminum, your largest consideration should be in the ability of the SYSTEM to maintain toolpath accuracy under high feedrates. Your cutting forces will be negligible compared to the force required to accelerate/decelerate the moving components of the machine.
In light of that, you should be considering the mass of the machinery components that you are moving, the required accuracy of your parts, and most importantly, the sophistication of the control...does it have the ability to "see" ahead in the toolpath and make velocity adjustments so that accuracy can be maintained? (and will the servos/drives be adequate to accelerate/decelerate the mass of the machine elements appropriately?)
Please don't be offended by this statement...but considering the magnitude of this project coupled with your knowledge, I HIGHLY recommend that you simply buy a machine. I really don't mean to sound rude, but I think you're setting yourself up for a lot of frustration and likely failure. If you really don't want to buy a machine, at least contact one of the OEM control system manufacturers (or retrofitter) to help you choose the right components for your mill.