Partner ACS and MailVS have a problem with the DND, it won't recognize the DND signaling and be acting weird. MailVS R5 can be upgraded with a patch all older ones are no good.
Having that said do you want to replace an old clunker with another old clunker?
Both systems are end of life pretty much and all Nortel products are going to be vanished within the next 5 years and replaced by Avaya products. Not necessarily a good move because Nortel has solid Key systems that are plugged in and running for years if nothing happens like lightning strike or bad power.
You should ask yourself if you want to replace the system again within the next 2-3 years or hang on to it until it dies, for the latter one buy another system for spares and also some more phones as there will be a time when you can not get them any more.
In 10 years your system will be good for the museum because the whole industry changes and 5 years is at the moment the expected life span of a system so plan well ahead.
Joe W.
FHandw, ACSS
Google it you damn kids