Cost of Central Air conditioner for 2000 sq foot home? We are looking to purchase a 1800 sq foot house that has...

We are looking to purchase a 1800 sq foot house that has 2 levels (a few steps take you upstairs to the main floor and a few steps takes you downstairs to the domestic room). The former occupants removed the C/A unit when they vacate the house (a bank foreclosure). What would a new element cost and could it be added into the mortgage monies since it was a fixture of the home that was removed?
Answers:    installatin around 4000,and unit u choose by yourself