Agree with Jazz on this one... Forget MDF, not only because of the strength problems but also it will move with time and therefore lose your precision.
Don't know about throwing it to the dog, I don't have one... LOL...

Are you planning on machining sheet material or blocks ? For sheets, a gentry / router type of machine would be best (and easily enough self built) but for blocks you would be better with a milling machine... More robust but not that easy to build, better start with an existing manual mill and convert it to CNC.

Welcome to the party! Enjoy...

RNR