I like the two column support, one of the toughest areas to create rigidity and who doesn't like to see innovation. I've long thought epoxy granite with fabricated steel structure (normalized) could provide a best of class performance that was also diy accessible. My thought had been a steel exoskeleton - the steel becomes the mold - but there may be complications with this i haven't thought of. On reason i like it was all kinds of bits and pieces could be welded on prior to normalizing whereas it might be more difficult to attach to the epoxy granite.
Were it me, I'd be thinking of the following.....
this is a big machine. big is great, but costs and time to construct go up in a non linear relationship . larger machines are required to make it and and purchased components are a lot more expensive. Of course the required work envelope varies by application, but your talking building a machine with the work envelope of a full sized mill - that's ambitious. Consider reducing the size - it'll still prove the concept and do excellent work - do you really need that large a work envelope? what type of shop do you have access to?
They other idea i've want to try is using Newall encoders rather than rotary encoders - this would mean going servo rather than stepper. Their accuracy is amazing and the mechanical aspect of rotary encoders is eliminated. Since you're going high end on everything, using these encoders and the accuracy they bring would create a three axis platform that could be used for just about anthing (a wire edi head?). Why wouldn't you go servo for a machine like this?
i look forward to future installments