I'll use Isel ball screws.
If you can read German I suggest you read the thread
"Optimum BF20 Vario gesucht, gefunden!" in Hobby Machinen on
www.cncecke.de registration required.
You'll find a wealth of information regarding the machine including mainetance, bearing replacement, ballscrew conversion, cnc conversion both with steppers and with servo's.
Their is a complete documentation of a ball screw conversion including photo's by Thomas74 that I recommend you read.
Erik Jan