You don't need to level up much to beat the main story. I did it at lvl 2. It's a great game. I've already become the Listener for the Dark Brotherhood on two seperate save files, one where I've beaten the main story, and one where I haven't.
the game starts getting hard when you level up to the 20s because there are about 30 beasts trying to kill you in the oblivion gate. it seriously lagged the whole time they tried to kill me. i luckily had some powerful spell scrolls. yea. well i am a breten so maybe that's why it's hard for me. AND i trained him to be a warrior.
On the file where I beat the game, I'm a lvl 6 High elf. And on the other file, I'm a lvl 2 or 3 Argonian. But once you beat the game, you don't have to worry about the gates anymore.
im going to try that with a new character. i just wonder who i will be. prob a orc then i'll name him shrek. or something serious.