Much of the notebook world is still proprietary. Though it's not impossible, it is highly improbable. I mean, ever mom and pop stores have their own notebook brand now, so someone sells the generic parts. Still, I've got to believe you'll pay more for the computer by buying separately.
If you want to learn how to build and service them, buy a pre-built, take it apart, and put it back together. That should suffice. Heck, Dell's site shows you how.
Regards,
Craig