The SX3 is mechanically the same as the X3 which has been done by many people. The only difference is in the spindle motor. We used to do a kit for ARC's X3's at one stage in fact I still have the original machine here.
The ballscrews from China, [Linearmotionbearings2008 on Ebay ] are fine as they are, coverting to double nuts is hardly worth it as with these samll desktop machine there is always a machine limitation and many spout off about microns etc but the truth is the screws will be as good as the machine.

You would be better spending to money on over spec'd drives to get speed and reliability, we ran these at 70 volts and they were very reliable.