I have worked on designing a light weight glass-epoxy-concrete machine for two years when someone who builds machines for a living pointed out that steel is way cheaper for the same stiffness, and that building in aluminium tooling plate is way easier than EG and also cheaper.

And are Gerry points out, you have to be willing to make lots of mistakes and correct them in order to build a good cnc.
You might even need to be willing to build at least 2 machines to get one that is more or less good.