I've worked with just about every intercom out there.
Of all the Nutone intercoms I've torn out and replaced it's always to bring them up to date and never because the unit is bad. I can not say anything bad about them other than they are ugly but that goes for 99% of intercoms made.
The linearcorp link to the M&S intercom is not a good solution for this job. It gains nothing over the Nutone units and will require drywall repair for fit. The features are the same.
The other issue at hand is wiring. I'm betting the existing system is a 3 wire with a sheilded door cable. It would be wise to re-use that wire. Cat5 is not widely accepted by most intercom makers. The exception to this is OnQ Legrand. Their intercoms run on cat5 and look good but would require a lot of holes to be filled.