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?
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.
Buy.
We bought a server from a local white box store. worst server we ever had. Built exactly to our specs, but the RAID card didn't like the Motherboard. Ended up costing us more than the price of the server in strange errors, testing, rebuilding, ordering other cards, and more testing.
Buy buy buy. A server is not a PC and the enthusiast/hobbyist in you is leading you down the path of wrongfulness. When you have real live users accessing them and relying on them for their daily work, solidity and reliability are crucial factors.
Go with Dell. Warranty is no issue when you need it. Get it for the length of time you want and the service level you need. Just had a drive fail in a storage server and we had it in 2.5 hours on a Saturday. With a custom bult unit there is often an issue over what has failed and who is responsible. You really want to avoid the finger pointing.
Also have had great servcie and suoppot on IBM and HP but Dell usually beats on initial price.
It is perfectly possible to custom build a server to rival anything on the market but you won't do so by buying cheap components at bargain prices. As the price difference for equivalent machines from a manufacturer or built from parts will be quite small, with the latter normally being more expensive, there needs to be a really good reason to go down that path.
When you buy a brand name server, and I'm not talking about some weird brand nobody has ever heard of before, you're (usually) getting a system that will have been assembled with solid components that are have been tested to work together. It will also be backed up be a decent warranty.
If you build your own you may well run into all sorts of issues caused by some of the components not playing nicely with some of the others. You will have no warranty on the complete unit, only warranty on some of the components. You can certainly forget about calling the manufacturer and having them on-site, with a whole set of components, within a few hours to repair the server with minimal downtime.
Buy.
The best locations I've seen so far have have been auctions from companies that have folded. I was recently asked to find a new server to match the cost and specs of a previous server purchase.
Turns out that the previous server was bought for a couple of thousand when originally it cost over 50 thousand. GBP here, but I think the difference is clear. Even the normal second-hand routes couldn't come close to the price of a sell-off after a company folds.
If you've had the pleasure of following the StackOverflow server build stories (podcast and blog entries), the lesson sounds like:
Just a simple problem with a ServeRAID controller not liking Western Digital SAS drives, but preferring the Hitachi brand. Never mind issues like: mobo/RAID card compatibility, mobo/NIC, power supplies, etc. I couldn't imagine trying to assemble a server together yourself with various parts, and having no warranty on the entire box as a single unit, onsite vendor service, etc.
Having the internals assembled and tested for compatibility are a critical advantage for the big brands.
Definitely, BUY them. The cost saving might look a bit tempting if you try to build it by yourself, but when you encounter a disaster scenario (and believe me, you ARE going to encounter that), you're going to save yourself LOTS of time.
Pros:
Cons:
Don't build a server yourself if you intend to rely on it for anything important -- If you're comparing parts apples-to-apples as it were the price differences are typically not huge, and the ability to call one vendor and get warranty service/support is almost worth the price markup the first time something breaks and you have to run around trying to get it fixed.
If you are put off by the price of Dell's hardware you may want to consider SuperMicro or other "whitebox" vendors - roughly equivalent quality, good warranty coverage and typically slightly lower price points.
Do you care about the system or will it be mission critical? Buy.
Is it easily replaceable and/or really simple (web server, compute node, DNS server)? Build from a SuperMicro barebone chassis.
Is your time worthless and so is your data? Build from all different parts.
I work in an academic setting so I often end up building (or spec'ing a beige box) to save money. It works but it's not great. SuperMicro barebones take the pain out of it mostly because the motherboard is actually good and for most servers you don't need other peripherals. They like 3ware RAID cards as well. All my compute nodes are dual Xeon 5300 2U chassis from SuperMicro... But if one failed tomorrow, I wouldn't be in a bind.
My core file servers are Dell and HP machines with 24x7 support contracts. That way I can get parts quickly.
My last experiences with a full custom spec'd beige box was a disaster... Vendor sold us a support contract and his only spare motherboard on hand was out of province (1 day delay) and DOA (5 days delay to get replacement)... The big problem with building is getting spare parts quickly. A sys admin I know had to wait 5 weeks for a replacement SATA backplane from Chembro a few years ago, 6TB of storage was offline until they got that part. Not Good.