This topic discusses using a Mac's built-in output:
viewtopic.php?f=3&t=16469
(In summary, the Mac's built-in output would be fine to use for getting started, but high-quality professional-grade audio interfaces will probably give a noticeable improvement in quality, as well as allowing you to use more than just a single stereo pair of speakers.)
MOTU's AVB models (such as the UltraLite AVB) might be good candidates to consider:
http://motu.com/products/
Here's a thread discussing audio interfaces, speakers, and Hauptwerk surround-sound sample sets and routing in general:
viewtopic.php?f=17&t=16236
therepetiteur wrote:I plan to, at least at the beginning, use a mac mini (i7 and 16GB RAM). I will use hauptwerk advanced edition with the large Budapest organ.
I don't think you would find a Mac Mini to be sufficient to use the Inspired Acoustic Palace of the Arts Budapest 'Professional version' (PAB Pro) organ in full, I'm afraid. The highest specification of Mac Mini that Apple make currently only has a dual-core CPU (3 GHz). The PAB Pro is particularly demanding in terms of CPU power and memory, so I think you would really need a minimum of a fast quad-core i7 or i5 model, with at least 24 GB of RAM (although you could potentially disable a few of its ranks to fit it within the free RAM on a 16 GB computer).
Amongst Apple's current range, to use that particular sample effectively in full I think you would probably want to look at a quad-core i7 iMac with at least a 3.6 GHz i7 CPU speed and at least 32 GB of RAM.