I agree with everything vablackwater just said. Mk 2 has a piston a thin plastic shim for adjusting the IP and a spring, possibly another shim on the other side of the spring. That is it in the way of moving parts. Very simple, very reliable, there is a reason so many dive shops use these as rentals. Don't forget how most renters/students treat the equipment, dragging it through the sand dropping it because they left their tank standing up...
you said you want a backup reg but didn't specify if its a backup you keep in the car, or as a pony, or what. You can pick up a MK2/R195 for about $250.
Yes Scubapro is discontinuing the X650. I dont know if they still have any but they were blowing them out with great pricing. If you can get your hands on one, its a great reg. If you can only find one at full price...Id pass. You can get an S600 which is smaller, or a G250 which is the same thing as the S600 but a bit bigger and should be a bit cheaper, the 555 is non adjustable, which is not a big thing but it is a real nice option to have. As the year goes on(you do have it serviced every year) and the adjustable orifice takes a seat(and the tech tried to give you the nicest lightest breathing reg), it might start to ever so slightly free flow, then all you do is turn in the knob a half twist and you're fine.