Bob, there are so many choices and variables here, it's scary. Ultimately you must mesh your budget with your goals, and understand that with a limited budget, your speed and accuracy goals may not be met. So in many cases compromise is the name of the game.
I was not familiar with the X3 machine until I looked at some pics on a site. It looks fairly capable, but the head mass appears significant, even counterbalanced with a gas spring. Getting tight, accurate Z I think will require you to forego the stock feed and install perhaps a Z ballscrew.
Cheapest: Fit adequate steppers in place of the handles, use stock screws. You must now deal with backlash, high forces, and fairly high torque requirement.
Medium: Replace stock screws with 5/8" or 3/4" Thomson-type rolled ballscrews and anti-backlash nuts. Torque requirement will drop, steppers may be pushed a bit faster, accuracy improves. Drawback - cost and time to machine the ballscrews and install decent bearings to keep the screws axially fixed.
High-end - Servos and high-grade ballscrews. Big $$, lots of time.
If it were me (just an opinion) I'd go for the X3 and convert it on the cheap in such a way that I could restore the machine later to manual. This would mean executing the "cheap" conversion described above, using the stock screws. If you feel adventurous and have a lathe or access to one, the medium approach with rolled ballscrews would be nice, especially since you can still, after the screws are in, restore the machine to manual, better and more accurate than it was, simply leaving the ballscrews in place. Thomson 5/8" rolled screws can probably be done for $100/axis using surplus BB.
There are others on this board with more experience than me who hopefully will chime in! Good luck!