Servers can be costly. You need to make sure your getting the right hardware for the job. That is what goes into designing a server. If you get the wrong Raid card all of a sudden your server can't read those 4TB hard drives because it only supports 2TB Disk sizes. You buy a 1gig nic, but easily surpass that all of a sudden you have a network throttle point which slows everything down. Do you really need a rack mounted server chasis? Should you spend a little extra money to get a 16 drive bay server instead of a 4U bay server? These are the types of questions that need to be answered when desiding on a new server. Should you design and build your own custom server? I've done both.
Some of the projects I've worked on.