To do this I have to complete all the Companions main quests. Do I understand correctly that doing so requires becoming a werewolf?

Yes, you must become a werewolf. I like the disease immunity so much that I willingly give up the well-rested bonuses from sleeping.

To elaborate a little, you don't need to get all the way through the Companions' quests to get Aela as a follower, only their main quest line. You also don't ever have to be a werewolf, except when you first get the beast blood. I never use the werewolf power, tbh. I just don't see a good-enough reason to give it up.

You need to become a werewolf, but can also cure yourself later in the quest-line if you so desire.

one of the things with Aela is her level on the main quest.

I have found none of the companions to be very strong.

Which means that i have to keep saving them from certain death.

just to let you know.

This is likely because most people encounter the Companions early on and at low levels. You can use the http://uesp.net/wiki/Skyrim:Wabbajack to scale them up to your current level (a level maximum of 50 for the Companions).