Your statement "in the plant" leads me to believe this is a large, open area, is this correct? I assume the units to be ~7,500-8,000 cfm a piece, giving ~1.1 to 1.4 cfm per square foot of area. 1 cfm per ft2 is suggested by ASHRAE for office space but seems woefully inadequate here. Processes, exhaust needs, and heat gains determine loads. Do the units return air and use an economizer or are they 100% outside air? How much process exhaust exists (i.e., is the space net neutral or positive)? How high is the ceiling - it might not be practical to condition the entire space when spot cooling could be used. 77 separate units (77 thermal zones) could find themselves battling each other without some sort of averaging control.
Also, six units per year replaced indicates an average useful life of less than 13 years for each. I tend to think you'd be better served by having a central chiller and boiler plant and eight well-made AHUs (~100,000 cfm variety each) serving the space.
These are initial thoughts; the best-fit depends largely on the plant processes and needs, as well as the company's performance and stability.