I would also consider how it has been cabled and as to weather it is 240V only or has a battery back up.
If it's 240V only, or doesn't even have fire resisting cables to the sounder circuit (let alone the detection/alarm ones) then I would say it must go.
Older system sometimes suffer from inadequate sounder & call point provision - found a G-1-2 floor buliding this week with only a single bell & call point directly off 240V no back up, indicator, or reset panel. (it's being replaced, but was in use until this year).
Sometimes these systems are not (or cannot be) properly maintained and have batteries in poor order and the control panels require replacing as indicator bulbs, switches and other functions are not working with no parts available.
In the right (limited) circumstances a 24V 'm' type system to the old CP1019 can be OK, although I would always advise an action plan for replacement over the years so that the budget is available - eventually it will fail beyond repair and you want to have the cash to replace it and not be suddenly stuck with finding £x,000 to keep your premises open.