Which Mac mini with 3 USB controllers can be used for high bandwidth USB traffic?
It seems that those after 2009 (which reverted to 4 USB ports instead of 5), only have 2 buses. The 2009 models are likely the only models with 3 USB buses.

If you get one of the newer ones with Thunderbolt, it's possible to connect to a Thunderbolt Display which would add a third hub. I don't know of any devices off-hand that will give you just another USB hub via Thunderbolt, but if they're not out there already, they're bound to show up.