You owe it to yourself. Pick a helmet that's comfortable. Your bike shop is the place to try different models and styles, and where you can find a helmet which fits properly. A too-loose helmet may not protect you. A too-tight helmet can hurt.
All decent helmets come with built-in vents.
Most of them are pretty light. And most offer aerodynamic advantages over a hairy head. As they say, it's cheap insurance. You don't need to carry a complete repair shop on the road, but you need enough little doodads to make some simple repairs. Buy a small saddlebag to carry this assorted stuff.