Is the owner someone that you know? How much is the purchase price? How many miles? Do you know what happen when "it over heated"? Is the CEL on, if so try to get the ODBII codes read.
Could be as simple as the cooling system needs to be replaced. Could be as bad as a couple of bent vales and other problems.
As far as an engine replacement?
seems like it would be easier to find a car that's not broke.