If ya down south, go poke ya head in and speak to Graham at Boss Differentials (Listed as Boss Automotive in the phonebook - Scarbourough Way Lonsdale)
He does all kinds of diff centres and diff work, and can suss it out and tell you how much ya looking at. He's also a seller for KAAZ mechanical diffs.
Service is second to none, and he's always willing to offer straight down the line advice. He has built many an old schooler diff, race cars, maintains god knows how many drifter's 31 diffs hahahaha, and tons of street cars
He'll be able to give ya an idea of the best diff centre for ya application and not let ya spend heaps of $$$ on something ya might not need