18' single swing gate is a monster. Think overkill for the entire system or you will be messing with it constantly. Make sure you use a large post (6x6x3/8" wall minimum with rebar passing through it at the bottom so it doesn't want to pull out). 4-5 feet into the ground. Large footing (at least 2' wide.) Use 3 of the largest barrel hinges available. You will need an opener that is rated for at least twice its actual weight.