A wet longsleeve will be COLD. because you have more surface on which water is evaporating.
anything you put on after the swim has to go on wet skin and that will be difficult. Arm warmers may work. A vest or zip jacket goes on easier although it will be less aero. who cares.

if you can't afford a wet suit right now think about renting. rent for the weekend before to practice swimming in it and getting it off.