Office condo unit AC leaking refrigerant

Our office unit in a condo is leaking refrigerant but because we own the unit, we are responsible for fixing it.

I was told that my AC is a McQuay model CCH 024A MFS.
The building manager recommended I change the whole unit, it's from 1989.

I was quoted $2500-3000 for just the labor but the other components are 20+ years old so that's why he suggested changing the whole unit.

I was told a new one costs $5400 plus tax.

What is the cheapest and best alternative in your mind? Can I get one off ebay cheaper?
Do I have to use McQuay or are there better alternatives?
And how do I know that the new unit will fit?

I can PM you images that I got if you want.

Thanks!