You best bet is to go LD-16. Those old operators based on the old Miami Elevator platform (HD series (All models)/early LD series, such as LD-03) are no longer in production, with limited mechanical spares.

For more info: https://verticalxpress.com/product-availability-notice-hd-door-operator

NOTE: This does not mean no parts are available still.

In my opinion, I think there should have been more checks and balances between the landlord and the sells staff.

As far as using GAL equipment on an old TAC serial, I would only consider it if I were changing my controller to something that supports GAL CAN (EC, Galaxy, Nexus) or discrete (i.e. MCE, TAC/ICON 32)