Does the current system keep up with the cooling needs, or did it at one time? If someone says the system is undersized a manual J Heat loss/ heat gain calculation to find out what is required for the house heating and cooling needs to be done. Any good contractor should do this. If you upsize the system that will mean increasing the duct size (generally replacment of the duct), and increasing the electrical service to the equipment. I would suggest American Standard, Bryant or Payne. Payne should be the cheapest and its just an affordable version of the bryant equipment. American Standard is the more economical version of Trane equipment - same exact product just different label on the outside.

We just had a new Rheem 15.5 SEER air conditioner installed in our Cape Coral home on the west coast of Fl. Our home is just a hair under 1800 sq ft, and we upgraded to a 3.5 ton because the home was built with a 2.5 ton builder model that was far too small, and could not cool that much sq footage. The install was around $3500, including increasing the size of the return duct, and adding another supply to provide adequate airflow for the larger system. The contractors we purchased the system from did a manual J, and were quite knowledgeable, and our replacement was completed in one afternoon, the new AC is working beautifully, and now easily cools the home to any desired temp without huge electric bills. I would question the need for a 5 ton and have contractors perform a manual J calculation, I'm definately no expert on HVAC systems, but it seems to me an extra 300 sq ft wouldn't need another 1.5 tons of cooling. Our system is from Rheem's value line of medium efficiency air conditioners, dosent have a lot of bells and whistles, but it does a great job of cooling and also dehumidifies very well for the very budget friendly price.
Dave