Buying v Building... Dell's are more expensive, aren't as upgradable, but come with a warranty. Building yourself is cheaper, but you don't have a warranty to fall back on. But you can reuse components saving you money on your next build.
No you still have warranties on components. The differences is instead of being able to go back to a single sources for you warranty. That source being Dell.
instead you have to go to the warranties on the parts at those manufactures. That easily means you are dealing with a lot more companies but the turn around time is about the same. Hell most are very willing to ship you out a replacement part as long as you put down a deposit which they refund when they receive your broken part.