I've checked preloaded brand name ballscrews on ebay, and found few solutions for Y and Z. I have to select ballscrew for the min possible diameter (inertia) and required stiffness.
Considering 180N load and 0.05mm allowed error, can I assume that total stiffness of the machine at the tool tip should be no less than: 180N/0.05mm=3.6N/um? Would then a 12mm dia ballscew with rigidity(K)=120N/um be enough?

I've sketched a solution for X axis base. Tubular structure would be made from a 50mm thick granite slabs, glued and fastened together with screws. Steel rail supports would be fastened on top with screws and epoxy, and then shaped and ground to create plane for mounting rails. Would a rail span of ~200mm be enough (I see that span used on gantry machines)?
Click image for larger version.  Name: 50mm granite slabs w steel supports rail span 198mm.jpg  Views: 2238  Size: 12.5 KB  ID: 27529