18,000 square feet? That's humongous!
I recommend you hire a competent mechanical engineer for a project this size. There's too much at stake with a structure this size for the HVAC to be done by hacks or "rule of thumb" contractors.
I visited a very large house under construction in Dallas awhile back. It used a chiller for the HVAC. Multiple air handlers everywhere, one chiller unit on the ground. Complex system, but apparently for the homeowners, budget wasn't a primary issue given the extravagance of the structure.
That house could easily be gutted and reappropriated for an office building...it's huge!