A load calculation is best IF you don't fudge input numbers to get the answer you want. Newer homes in our area can be as little as 1,000 sqft per ton, 800sq ft per ton is common. Older homes and upstairs units can go as high as 350sq ft per ton. Unfortunately "rule of thumb" 500sq ft per ton is what typically gets installed regardless of actual load...

On the heat side pick the smallest capacity furnace that has the blower size you need for A/C, it's more than enough heat for just about any southern home.