When we got ours, Withams wrote the chassis number incorrectly on the MoD Form 654, so we had to get a date into service independently. If you write to DE&S they will provide you with a date into service from army reg and chassis number but I'm not sure if they could tell you the build date. Ours was first in service early in 1986 so could quite conceivably have been built earlier than the cutoff, but I don't know how you would choose a particularly old one! The military registrations are issued sequentially so you could get something in the 85KDxx/86KDxx range and hope for the best? (Ours was 87KD11).
That said, it's not a big job to add side bars and it's a one-off cost for peace of mind. Ours were done by protruck - very professionally - maybe give him a ring?
Stone