[Capricorn]

What you need is a burden test. Meaning that you need to know what your heat load is. The size of your windows, whether single or double paned makes a significant difference.

The burden test takes your R values from the attic, walls, floors, windows, etc. The R values are composed of insulation values and what they mean. This is the only true way to size an A/C appropriately.

Any legit Company will perform this service for free.
Tell the company that you are searching for free estimates, if they are not willing to do so, shine them on!

I have been doing this kind of work since 1988.