Another comment mentioned this already, but a single low end PLC for the central control unit (such as https://www.automationdirect.com/adc/overview/catalog/pro

Another comment mentioned this already, but a single low end PLC for the central control unit (such as https://www.automationdirect.com/adc/overview/catalog/programmable_controllers/click_series_plcs_(stackable_micro_brick) https://www.automationdirect.com/adc/overview/catalog/progra... ) plus RDCs at each building would cost under $5000 easily, all while being in compliance with any standards governing such equipment.