There is no way to accurately answer this question without a proper home evaluation. In a 2000 sq ft home for example, a 3 ton unit may work great, while another 2000 sq ft home may need a 3.5 ton unit. Yet, another 2000 sq ft home only need 2.5 tons.
If your AC runs every day, replace the filter at least once per month. If you have many pets, you may need to replace more frequently. During cooler months there may not be a need to replace the filter. A simple check of the filter will answer your question.
There are places on the internet selling AC systems at reduced prices. Keep in mind, most manufacturers will not warranty an AC system that is not purchased and installed through an authorized dealer. Buy locally from an authorized dealer for best performance.