Hi P2N - Extremely optimistic project. Combo machines are compromises towards one of the functions. In this case you are building a Mill not a router due to the mention of 20mm aluminium. You need to forget about construction extrusions and use steel or aluminium structural sections. A 1500mm wide gantry needs to be very stiff to do what you describe. At a guess you will need a steel 200x200x12mm thick square section at least. I suggest you find a commercial machine that does what you expect and this will point the way to dimensions and materials of construction. You will need 16mm plus screws to side step whirling (or whipping). Your process goes like this: take the best paper design you think you have and get it into CAD asap. Ignore all motion items I suggest that 20mm or 25mm square rail will be fine. 2) do structural analysis and aim at 20N/um static stiffness with spindle in the middle of the gantry and full down 3) Once you have the machine at the correct stiffness then you know the weight of things so can do the motion calcs to determine screw sizes and bearing sizes. At this point various bits won't work out as you thought so you then fix all the loose ends and changes of minds and you end up with a working machine on paper Then you cost it fully. Then contemplate the cost and how you could spend your money better. Then after that review A) start again as its too many $$$ or has drifted from your original ideas B) commit to the costs and order everything and start building... budget about 6-12 months to get to this point. Done this several times so know how it goes.
The shortcut is to find a thread with a machine that you like and follow those thoughts and clues. Saves a bucket load of time and effort. Keep at it, Peter