That mill looks a little more robust than I originally envisioned. I think it will work for a small hobby machine. One thing I would do is mount the motor for the spindle on some riser in that square cutout section on the top. This will put the mass of the motor closer to the column and if it's filled with EG it will make a good vibration dampening mount for the motor. You may also wish to consider beefing up the base and the bottom of the vertical column to add some mass and stiffness. It need not take up much extra space but it will improve the machining quality.
If you have access to some surface plates and a mill you can get it pretty square. You can machine the parts very even and square and then use bolt together construction and then pour the EG in there.