Buying new vs buying used has its downsides.
1. No warranties. Most likely the warranty has expired on every part in there. There's no guarantee that the used parts will work very long/at all. Buying a used hard drive is a pretty bad idea for many reasons. Average failure rate only increases as time passes. If more than 3 years, I don't think it's worth it.
2. Tech Support Costs. You will need to do your own tech support or pay someone else to do it. There's no Dell tech support which will remote in and check things out or come down to your office if there is a problem. You will have to foot all hardware replacement and IT costs.
3. Legacy pricing. I'm sure many have noticed that when you were upgrading older computers with more RAM (DDR or DDR2) that it's almost double/triple the price of DDR3 for the same capacity. Those older processors are also more expensive (and slower) if you want to upgrade.
4. Limited and expensive upgrade options.

All of these things tend to cancel out the price savings of used hardware. If you can find used but relatively new stuff, you're in good shape I guess.