If you're looking to stock up with spare parts, it may be worthwhile to get an extra rear derailleur or two. From my time working at a bike shop (many years ago, admittedly), rear derailleurs get buggered up much more often than front ones. In fact, it was quite rare to replace a front derailleur, whereas rear derailleurs usually require replacement from crash damage or simply normal wear and tear (they get shifted much more often and have much more movement).