It sounds like you need two different machines: a mill for the aluminum copper and PCB work, and a gantry router for other things. With your limited budget, you're not going to be able to build both of them simultaneously, and making either out of 80/20 extrusions will exhaust your funds before the build is done. If you can weld, consider building as much as possible out of steel, which is relatively cheap, and using aluminum solid stock for the moving parts that need to be light.