CNC Router/mill ideas

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Hi fellow /diy/ers, me and my father are going to build our first cnc. We want a working area of 130x100cm (rougly 3x4 feet). I want a fixed gantry design, while my father wants a moving gantry. And here comes the problem, we live in the third world (argentina), and cnc supplies are not cheap. We can get SBR20 rails for a good price, but SBR25 are more than double the price, which is a no-go. We can also get hiwin-like 15mm rails for about the same price (a little more, but not much). With hiwin-like rails we need a very flat surface, that means aluminum extrusion which is super expensive here (we don't even consider that route), or a machined surface. We have a mill and a shaper, both of which only machine a 50cm lenght, and sending our pieces out to machine would be too expensive here. So, we need to go with SBR20 rails, which don't need that flat of a surface (I belive that with hand selected structural steed, properly squared, aligned and mayble shimmed, it's doable).
We want to cut mostly wood, with the occasional aluminum (I don't care if it takes an entire day to mill some aluminum, but I want to be able to). What do you think? Is it doable with structural steel and SBR20 rails? Moving or fixed gantry?