One thing I find odd about all those ATC cartridges, is I've never seen any threads where people have actually used one. I've certainly never seen anybody complaining about them anywhere, so I'd assume they must do their job without any issues.

Another option to consider, is there a few Taiwanese spindle manufacturers who do built-in (aka with motor built in) spindles, however I've got no idea on pricing.
POSA, and Royal Spindles are two that I can remember, but I'm sure there are others.