umm... if the oil looks like a chocolate milkshake dont buy it.... if its dark brown that is bad, and my D16Z6 never smokes at all durring any race or RPM. My A/F ratio stays the same untill I hit 3200 and then both the air and fuel mixture go up, and when that happens there is no smoke.... If you are geting the engine for 400, I would replace the stuff that is easy to replace while the engine is out of the car, like the head gasket, timing belt, water pump, and check the valve seals, they may need to be replaced. Also check the condition of the coolant, what color is it and ask him if the engine has ever over heated?