Well, the newer the used Mac Pro is, the more likely it will do what you need. Buying a used one is a little scary if it's not under warranty anymore. My Mac Pro died on me once and incurred $1,000 worth of repairs. Fortunately, it was covered by the AppleCare warranty. It's five years old now and I actually just sold it for $500.
Jason Jenkins
Flowmotion Media
Video production... with style!