There is a "new" hardware platform currently in distribution which will be required later next year for any site desiring to move to release 9.1 and beyond. I would be very suspicious of anything on the used market today that would offer that level of future upgradeability.
As for a hardware spare, something to have on the shelf, yes that'd work though you'd need to move over your sysid from one system to the other then restore your database. Sorry if I misunderstood the question. This spare would also be limited in its ability to be a swappable system without a lot of busywork swapping T1/E1 modules, FIMs, etc.
Two most likely items to fail are the hard drive and, if used, the internal AMB. I can change either and have the customer back on line in less time than it'd take to breathe life into a spare unit from the cabinet, unless the spare was literally identically configured and all ready to go except for the database & sysid.
I just hate to see people buy a pig in a poke.