My thoughts on spindle for milling steel are you needs lots of torque and not so much speed. Popular ways to do this are a pulley driven spindle rather than direct drive. Maybe 1kW or so motor into a 2 speed pulley arrangement to drop the speed and increase torque.

3 phase motor into VFD to give infinitely variable speed range would work.
Can also be a servo with encoder if you need rotary position for rigid tapping.
There are also DC motors using variable speed control. I prefer VFD control over these as the inverters give lots of control options and possibly more torque (?)

Z axis alignment to bed is usually done by making the whole column a single unit welded onto a flange. This is then bolted onto a receiving flange on the bed which allows shimming or you can go for epoxy compounds but these are pricy !