Quick SketchUp modelling really helps in putting all the pieces into perspective. The frame will be a mix of metal and wood. In the sketch you can see a brushless spindle for reference (replace with your weapon of choice). The spindle mounting wheel sleds could be flipped to go inside the Y-axis rails making the setup more sturdy by sacrificing Z height. In fact, this model better illustrates that the frame is great for 3D printing in this configuration because the Z-axis has plenty of range.

I still need to order the Z-axis lead screw setup. I think I'll go with a 20-30cm lead screw for height.