Will, start out with something really basic like a desktop router. Forget limit and home switches, play with belt and lead screw designs. You can try out different arrangements based around the same basic frame, add bells and whistles as you like. Get a grip on the software side, see how much force your drive can produce (luggage scales) then move onto a mill or lathe as the fancy takes you. Even start with just one axis !!! Most of all, have fun playing.