I am new to the forum so please be gentle

We are in the process of buying a home that does not have central AC (yet). It's about 6000SF located in the Northeast.

One of the issues I've had in the past was with a plumber that "installed AC systems". To my chagrin, this has resulted in mediocre performance of my AC system. At the end of the day, it takes forever to cool the house down because the "professional" didn't size my units properly (confirmed by others who've serviced my equipment).

I've also been told there's a world of difference between HVAC pros and plumbers that install AC units.

I'm looking for some help if finding people who are truly great at HVAC and can install and stand behind a 6000SF retrofit where the installation will be properly sized and calculated for the environment. Are there certain questions I should ask? Are there certain "red flag responses"? I'm all ears and thank you for taking the time to read this.