Is it going to be similar in size to a sieg x2 mini mill?
Will you use dovetaled slides? Have you considered buying a chinese cast iron cross slide for xy wich you could adapt.
I have been thinking if it would be easier to get rigidity and precision in a mini mill, configuring it horizontal, instead of verticaly, perhaps not.
Have you seen this:
https://s.click.aliexpress.com/e/bUsfSI4p