I'm trying 540 for a bit of a challenge, having raced Modified touring for the past 7 or 8 years(offroad for many more before that).
You'll generally need to modify most cars to run the ratios needed in the 540 class. It's a fact of life. I've just picked up a mint TC3 to run for $160 and it's in excellent condition. The car had been modified slightly by it's former owner to allow the use of a 40t 48 pitch pinion on the Taranaki circuit, which is one of the longer ones(not as big as Ruapuna though)
Up to 40t 48 Pitch pinions are easily available. Hobby Direct in Auckland sell the Yokomo ones for starters. KW Racing in Wellington can get them too.
Kelvin also retails The Yokomo, X-Ray and and Corally cars(NZ dist)in the lower North island. He is only a phonecall away. If you want his phone number please PM me.
You may wish to add your location too and I can give you the names, web addys etc of a few other places and people to try for information.