Hard drives are so cheap that buying a new sata one in a few years really isn't a big deal. Also in a few years SSDs should be commonplace, so you wouldn't want an old rust frisbee anyway.

Also, I am a great believer in buying entire new computers, rather than building a new one and scrounging parts from your old one. Modern computers, even if they're 4-5 years (or more) old are still perfectly usable for low-end office and web-surfing tasks (and playing online flash games), so you can give them to a friend or sell them cheap on ebay. But if you take the HDD out of one, you've only got 'most' of a computer left, but not enough for it to be useful by itself.