Looks like the Southern Festival of Speed isn't until Feb
(
http://www.dunedin-tourism.co.nz/sfos/). Not sure about your Previa but I think the 'speed' bit in the title rather counts it out.
Your circular route down to Queenstown and back via the west coast looks good, you get to see some really contrasting scenery. Take a raincoat for the West Coast, it can be very wet and look out for speed traps, they are everywhere. You don't even need to be speeding these days! My brother was recently stopped and fined, points on the licence etc for 'failing to stay in lane' in other words straightening a bend, even with ample visibility!
You are welcome to stop by for a beer as you head back to Chch after crossing Arthur's Pass, just drop me a PM and I will give more details.