Erm your a talented Man and I would love to meet the Model riding that beast. . Lol

Don't worry about what ballscrews and motors yet because you have much more to do before you'll need to know that.? First you need a machine size/design and then choose material to build it from which will be needed to answer your ballscrew and motor question. When you have setteled on a size and design then you can choose the screws and motors to suit it.
Worst thing you can do is go buying blind before doing the home work. Just like you plan the Ice sculptures and they have a process how they go together in order building a CNC machine is same there's an order to build it in.? Starting with design again like your Sculpture's.!!