Small 2 Bedroom, 900 sq. ft. Home HVAC System Questions
In general, 500 sq ft per ton, so you need 2 tons. but it all depends on how good is the house insulated. like said, load calc is a good start. If I were you, I put the air handler in the basement which will have both evaporator coil and heat exchanger, and put the condensor outside near the air handler. Flex duct is OK. You also need to plan the return air duct..size , location, etc.. there are lots of design work. have you thought about heat pump ? may be you need to discuss the whole thing with an A/C tech before you start doing anything.