i picked up a used clone a few weeks ago to do basic jobs but have done quite a bit of research on them all. It looks like you have the one with the upgraded spindle motor and a belt drive up grade which i think is the first mod people do. The good thing about yours is the fixed base as this is a little more solid than the adjustable one however a lot of people seam to go for building a hole new steel frame to bolt the base and the column to to get it even more stable. there is a good thread here about it and is what i plan to do
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http://www.homemodelenginemachinist.com/showthread.php?t=7479

youtube has lots of cnc conversions and as mentioned there is a kit from cnc fusion which uses ball screws all round. Again i have been looking into ball screws but ideally you want one with a small ball nut which i have not found yet.

another common mod is to use ether a gas strut or counter weight on the z axis.