hey mate, this is going to be a wicked project, can't wait to see it as it progresses.
pc in muscle car = epic .
a thought on vibrations: i don't know anything about mac mini's but make sure you mount the pc so that the HDD is vertical. this gives long life as bumps and vibrations are less likely to cause the spindle to impact on the platter. otherwise consider going solid state drive maybe? i know they are expensive, but a typical HDD is the most vulnerable to vibrations and ssd's are basically indestructible hey.
looking forward to seeing more!