bst740,
Bad service can kill any device very fast
IMXO, in your case good service is a limited intrusion into the system.
I've more than hundred MD’s LIMs on the net installed 89-91 years.
They have current versions as BC6-BC10.
And I don't see ANY difference in reliability between them and new Consono systems.
The only problem in old MD could be DC/DC converters.
If you have no spare - don't switch them off under any circumstances!
But you will keep this problem forever, even if upgraded to BC11.
Ensure good and reliable power supply, fresh batteries, grounding.
Old digital phones, extension boards, HDU2 and another staff should not be a problem to find as second hand in any country.
Regards