For things like a Mac mini (with Thunderbolt), you don't need just a 2009 27" iMac. You can do it with any 27" 2009-2012 iMac since Thunderbolt devices (Mac-wise) work fine. For non-Thunderbolt connectivity to the iMac, that's where the 2009-2010 27" iMacs have something over their later iterations.