So your plan is to make positive forms in sections that you can pull molds off of in a material that can handle hot glass being poured into it? I'd like to see pictures of this process, once you've got it working. Is there a reason you have to print them and can't carve them with a CNC machine?
As for the printer you want to build, I'd start by looking at large-scale printers that exist already, like the Gigabot X: https://re3d.org/gigabot/ I like the idea of a printer that can use pellets instead of filament, since filament is not just expensive but a hassle to deal with. A large extruder nozzle size also helps when making large parts. The frame isn't too different from that of a CNC router, and it uses profile rails, but printers tend to use belt drive rather than screws or R&P. Speed is emphasized more than rigidity, although more stiffness doesn't hurt. I'm not sure if anyone is offering large-size printers in kit form, but you could probably build your own frame, provide your own motors, motion control parts and electronics, and just buy the parts you can't make yourself economically.