When I setup my Taig mill I was thinking the same thing. I was told not to worry about it. Does not matter what they think, when you cantilever that much weight off the side of that little head on a Taig you are going to have more friction and wear on the slide and screw.
So I made up a big pulley, put a ball bearing in it and screwed to the a floor joist over the mill. Run a string from the motor over the pulley to a weight equal to the motor.
Works great and lets me run the Z a little faster then without the counter weight.
Sounds like you are using Taig parts on some other type of mill so this may or may not work for you.
Ultimately I want to put a brushless motor on it the kind used in remote control boats and such. Most who do this use smaller very high speed motors and a belt and pulleys to the spindle.
I am thinking use a motor intended for larger RC models as they tend to spin slower and be a better match for direct drive and should be smother and quieter. Down side is more expensive.
The stuff I want to look at is Castle Creations speed controls and NeuMotors.