is it 1.5 gallons or 2.5

Once you drain the radiator, refilling will probably be around 2 gallons. Buy a jug of Dexcool and a gallon of distilled water and use equal amount of each to get a 50 50 ratio and add till the min line on the reservoir. Drive the car to get the engine warm and shut off and cool down and recheck the level and add as necessary.