A Mac Mini would be fine.
All Macs come with the XCode developer tools which are the core development enviroment for OSX.
To develop for OSX, you'll then need to sign up for the iPhone Developer Program -
http://developer.apple.com/iphone/program/ and download the iPhone SDK -
http://developer.apple.com/iphone/program/download.htmlJust make sure it is an INTEL mac .. not an older PPC version.