The 900QMD uses the AP 9000 quartz micro drive. Same one on my AP600QMD.
AP says they no longer have any stock but they have the older 8001 which works but doesn't micro-step hence the tracking is less smooth.
That said the 9000 QMD controller is fairly basic (I've cracked open mine to do some repairs). Wally from AP will repair it if it breaks, or basically most competent electronics guys can repair it.
The old 900QMD can have its declination axis upgraded so it becomes comparable to a 900GTO, but AP does this very rarely and only if enough QMDs come in for the service because it is a loss-leading service for them.
The 600QMD (and I imagine the 900QMD, since it uses the same controller) uses a very common Nippon Pulse stepper that is also used (with a different gear ratio - 120:1 instead of the 150:1 in the AP) in the Vixen MT-1 motor. So the thing is eminently serviceable.
In my case I put a German Littlefoot drive on my 600QMD, so I have GoTo. Had to replace the motors as well.
I was almost going to buy the AP900QMD that went for $3750 a month ago on the other site. That one was pristine, and drool-worthy. But... word is that the 900QMD without the declination upgrade is "similar" to the 600QMD in terms of DEC guiding performance - and I want to get away from that DEC guiding performance.