Zapp Automation are good for steppers and mechanical components. Drivers are a bit pricey IMHO but I'm sure excellent quality. Quick look on eBay shows you can get a BoB and 3 stepper drivers for about 75 quid. There are also a lot of suppliers of suitable psus on eBay. I fitted my Denford Novamill with new electronics for significantly less than £200 mainly from eBay.

On the question of strength of the mill, with cnc because the machine does the work you can use smaller cutters running faster which I think makes less demands on rigidity. But having a round column without a gib will makes tool changing a pain and really only a lead screw gives good z dimensional accuracy, rather than a quill feed.

Backlash is a pain, and in my experience the backlash compensation in Mach3 only works well for very small amounts of play. You may think that your screws don't have much backlash, but I think you will be surprised if you measure it. My manual VMB and Super7 both have about 0.2 mm backlash in their leadscrews - I've cnc'd the lathe with a ball screw on the x axis, but I need to convert the z axis, as it's a pain having to plan some cutting sequences to compensate for it.