i dont know the extent of you knowledge but very generically speaking, people here are handwheel-phobic.
They use 5/8 5tpi ballscrew for x and y axes on benchtop mill conversions.
3/4 5tpi for z axis
Double angular contact bearings on one side, unsupported on the other.
buy ballscrews here in order of affordability and, industrialhobbies.com homeshopcnc.com mcmaster.com
Motion drives here. geckodrive.com xylotex.com, for the size of your mill the xylotex 500 oz/in is plenty enough torque, dont quote me on that, see my other post on torque.
when someone cnc's a knee mill they usually go for the quill because the knee weighs half a ton, you dont have this problem.