This is the canonical question for "Should I build coomputing hardware myself?" questions.
I have put together countless PCs, but never a large server. The geek in me says build it, but the realist in me says let the manufacturer handle it when there is a problem. Ignoring the time penalty involved with the initial assembly time of a built one, which is a better solution? Have you ever run into a problem with a home build server that would have been solved easier/quicker/cheaper by going with a manufacturer? Are there any features that manufacturers give that aren't easily attainable with a home built server?
Best Answer
Buy them. And buy them from alternative sources if you need to be frugal - Craigslist, Ebay, Dell Outlet, etc.
If you end up building them - go with SuperMicro - great gear.
But Commercial Servers will have better out of bandwidth management, better systems management, better support, etc.
And if you need to pinch pennies - use third party memory (i.e. Crucial) - its cheaper and just as good.