It sounds like what you really want is a Vertical Machining Center. Most of us here are building routers for cutting wood and similar materials,but they won't cut steel very effectively, if at all. While it seems like you're well set up for building a router, and probably could build one that would do an acceptable job with aluminum, if you're really shooting for steel, you need a much heavier and stiffer machine than you're going to be able to build yourself, unless you've also got an iron foundry in your garage. Your best bet would be to keep an eye out for a used VMC with a broken or obsolete controller. Often machine shops will sell them for scrap value, just to get the space back that they were occupying.