OK, I converted a round column also, and will throw my .02 in. For ball screws, I use Roton 5/8" and have had no problems with them. 5/8" screws are "more common" and therefore cost less!
Steppers I can't really help you with, but based on threads I have read,probably will work fine. Others may give better opinions.
Mach 3, a lot of people like and it seems to do the job, HOWEVER, due to many problems with laptops, I understand they are not supported nor recommended. There seems to be a lot of problems with power saving and sometimes timing. Also, if your looking at using the usb prot instead of parallel, you need to consider something like the smoothstepper. Again, I don't use Mach nor a laptop on my system so I will shut up at this point.
For cad, I use progecad (http://www.progesoft.com/) which is free for non-commercial use. For Cam, I use Sheecam (www.sheetcam.com) and it works very well for my purposes. For this program, it is best to have a cad program that outputs rel12 dxf files for the most reliabliity.
Hope this helps.