a. The general standard is for every 400 square feet of livable space (not garage), you should have a "ton" of refrigeration. A ton is equal to 12000 BTUs (British Thermal Units). For example, a 2000 square foot house requires a 5-ton AC system. This is just a rule of thumb. A heat load calculation may be needed in certain situations.
a.Your AC will always be electric as of now. If your home has access to natural gas it is a much more efficient and effective way to heat your home.
a.The best way to heat and cool your home is determined by numerous factors. I recommend consulting a licensed HVAC contractor and evaluating your utility provider.
a.For the life of your AC and heating system, I recommend having your system serviced twice a year.
a.I recommend changing your filter once a month in the summer time and once every 2 months in the winter time (if in the valley I.E. phoenix metro area). If you have sensitive allergies, please consult an HVAC professional.