Like if you wish to use cycle for office commute you choose road cycles which suits for plain roads, pavements most of the leveled surface. Or if you like adventure climbing mountains or off road drive mountain cycles will be apt. Or your clueless what to choose try hybrid cycles it will fit almost all the use case.

Bear in mind that if the helmet did its job most people would tell you that they did not even hit their head, or did not hit their head that hard. And the thin shells on most helmets now tend to hide any dents in the foam. But if you can see marks on the shell or measure any foam crush at all, replace the helmet.

The bottom line: Leaving your bike outside for a day or two won't do major damage. You may see signs of rust after a week of neglect. After one month in bad conditions, your beloved bike parts will start to degrade