My unit had a Bridgemill with 4 axis servo controls and it served us well. When I came home, I found a used Shopmaster 2000 with Quadra lift. I am now working on adding the bridge mill style support to upgrade the machine. Since you already have experience with your machine, you probably know its capacities and limits. Do you have the Quadra Lift attachment? As a CNC lathe it should work fine as is., but for CNC milling you really need the quadra lift and bridge arm support. These are the things that set the Shopmaster apart from the rest of the 3 in 1 machines. Our servo unit was powerful and fast- 150 inch per minute traverse. I think most stepper systems will be in the 30 inch per minute range, which is probably adequate for most hobbyists. I would recommend DRO if you don't already have it. Ball screws will also help, but are not absolutely necessary, on my 2000 machine I have adjusted the double acme nuts down to about 0.002-3" backlash on X and y. On the Z, the ball screw upgrade will be a big improvement for any close tolerance CNC milling.
Hope this helps