Here's my opinions, from general reading of forums etc. Bear in mind I have not converted my X2 yet and have no experience with home CNC.
I plan to use Gecko 201s and the 400 oz.in steppers from homeshopcnc. I understand people have used the smaller Nema 23 steppers, such as those that come with the Hobbycnc kit, but I am not convinced they will give the performance I want with standard leadscrews. People have reported good performance, but almost always with ballscrews and I don't want to use them initially. I figured I could overcome the extra friction with the bigger motors, which could also be used ono slightly bigger machines if needed.
I'd also be interested to hear opinions on this from more experienced users.
One of the biggest issues with the stock X2 mills, is that the lead-nut slots, in the saddle castings, are not always machined true / square/ parallel to the dovetails. If you can get the saddle set up on another machine and re-cut the lead-nut slots true to the dovetails (be sure to indicate the major dovetail prior to any cutting) than you will not need to worry so much about motor size. When I started out cnc'ing the X2 some 3+ years ago (I have provided countless kits and full machines since then) I used 116 oz pac. sci NEMA 23's and was getting 20-30 IPM rapids with stock lead-screws using direct drive. This was an improvement over a 2:1 pulley ratio I tried. With step motors, you are better off with direct drive or else a 1:1 pulley ratio if you plan to use belts. The torque curve of most steppers combined with the fine pitch stock leadscrews will require higher motor speeds than a ball screw set-up. Remember that as the rotational speed of a stepper increases, the torque output decreases.
If ball screws are within your budget, then they are definitely worth doing. With my newest ball screw kit offerings, I utilize a yoke that attaches both X & Y ball nuts to a common mount. The mount is then bolted to the @ center of the saddle. This approach does require some saddle stock removal to facilitate clearance for the X nut and the yoke itself, yet does not compromise the saddle's integrity. This is one major advantage of the X2's design over say the X1 where the Y axis nut is bolted to the base casting thus inhibiting any possibility of a yoke type mount.
On another note, Homeshopcnc's 400 oz motors along with Gecko 201's will provide for a great cnc conversion of your mill. I just installed that very combo on the Z axis of a new mill I am developing. I will most likely be updating the X and Y axes as well. The amount of cutting force you will have available will be more than enough.