Alex wants to build his own computer. Leo says that building his own PC is a good exercise and he'll learn a lot about how a PC works. He won't save any money doing so, though. And if there's something wrong, there's no one place to call. He'll have dozens of different parts and the manufacturers will just pass the buck. So he'd end up being his own support.
If he does want to do it, he should check out PCPerspective's Hardware Leaderboard. They will describe four different levels and within it, they'll tell him the best parts to get. Then he can buy them from Amazon or NewEgg to get the best price.