My Nissan needs exhaust manifold gaskets needs replacing please help?

I have 300zx 1990 and 118k, bought it from friend he gave me exhaust manifold gasket set in a fel pro box. How much will it cost to replace and will it be easy? It is pretty loud and small smell of gas around whole car after it sits for 8 hours. ?? Any help will be appreciated, i called a mechanic he said do they both need to be replaced? I dont know but all my friend gave me was a set of gaskets and 1 set goes to each exhaust right so i should only need one gasket (set) replaced?