I need at least 24k+ RPM
Don't know about
need but faster is better (unless you like boards taking forever to mill). There is a limit to how fast you can go which is determined by the cutting tool material and the stuff you're cutting through, which is measured in Surface Feet per Minute (SFM):
http://en.wikipedia.org/wiki/Surface_feet_per_minuteEssentially, it's a measure of the maximum speed at which the edge of the cutting tool can slide along the material you're cutting. Since the speed depends entirely on the diameter of the tool, the smaller the tool the faster you can spin it (and the faster you can push it through the material). When we are talking diameters of maybe 10mil, the limiting factor is not SFM as would be normal on larger tool, but how fast you can physically spin it. Thus a 24+krpm spindle is not
necessary but
possible to use and desirable.
However, the faster you spin it the more trouble you're likely to have with lubrication and runout. Swings and roundabouts.
As for your requirement to do SMD, you need a) a very fine cutting tip, and b) practically no runout. Suppose you have only 0.1mm runout at the cutters tip: that is half your pad to pad clearance for 0.5mm pitch pads. To stand a chance, you'll need a 0.1mm cutting tip (aka 4 mil) which is going to be a) expensive, and b) prone to breaking.
So, if you get a choice, go for bearing quality over speed. A really good one is unlikely to be cheap.
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