If you want your smaller machine to cope with stainless steel I wouldn't even think of making it yourself if your ideas are that it's a sort of gantry router. You could get a standard vertical mill of appropriate size and convert to CNC by adding ballscrews, steppers/servos and a controller. Such a mill would also be useful to build the larger machine, so you might want to reverse your order of construction!

I'm trying to work out what you mean by 5 axis for your gantry router that you want to cope with 8x4 sheets? Normally you have Z (up and down) and XY - leftright and forward backward - 3 axes. That will cope with cutting shapes out of large sheets. Other axes may be added but how useful they would be in what is basically a sheet-oriented machine I don't know - what are you trying to cut?