When you've owned an OEM system and done a few upgrades, you'll get more of an idea what goes where and how. Not being an expert, and even though I have built my present system up from the ground, I still don't really feel I would have enough knowledge to guide someone else on how to do a home build from scratch. Having felt my way along, I can assure you it's not difficult, though most of what I've learned is through upgrading and reconfiguring an existing system. I think if you are unsure then this would be the way to go, as it teaches you a lot.
There are books available on how to build your own, so in the meantime, why don't you invest in one and see how you go from there?
Hope this helps
Dr. V